Biology SL

1. 1.

   Distinguish between the processes of mitosis and cytokinesis.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that mitosis is the division of the nucleus, in which replicated chromosomes are separated into two genetically identical sets. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that cytokinesis is the division of the cytoplasm, which follows mitosis and produces two separate daughter cells. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that mitosis alone produces one cell with two nuclei, whereas cytokinesis completes the formation of two separate, independent cells. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Mitosis divides the nucleus; cytokinesis divides the cytoplasm. They are distinct, sequential processes even though they are often described together as 'cell division'.

   Marking points

   • States that mitosis is the division of the nucleus, in which replicated chromosomes are separated into two genetically identical sets.
   • States that cytokinesis is the division of the cytoplasm, which follows mitosis and produces two separate daughter cells.
   • States that mitosis alone produces one cell with two nuclei, whereas cytokinesis completes the formation of two separate, independent cells.

   Examiner tip: Mitosis divides the nucleus; cytokinesis divides the cytoplasm. They are distinct, sequential processes even though they are often described together as 'cell division'.

2. 2.

   Marking analysis: A learner attempts the following task: “Distinguish between the processes of mitosis and cytokinesis.” Their response addresses only this point: “States that mitosis is the division of the nucleus, in which replicated chromosomes are separated into two genetically identical sets.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
   2. Requirement 1: Recognises credit for the stated point: States that mitosis is the division of the nucleus, in which replicated chromosomes are separated into two genetically identical sets. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   3. Requirement 2: Identifies the missing requirement: States that cytokinesis is the division of the cytoplasm, which follows mitosis and produces two separate daughter cells. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   4. Requirement 3: Identifies the missing requirement: States that mitosis alone produces one cell with two nuclei, whereas cytokinesis completes the formation of two separate, independent cells. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

   Marking points

   • Recognises credit for the stated point: States that mitosis is the division of the nucleus, in which replicated chromosomes are separated into two genetically identical sets.
   • Identifies the missing requirement: States that cytokinesis is the division of the cytoplasm, which follows mitosis and produces two separate daughter cells.
   • Identifies the missing requirement: States that mitosis alone produces one cell with two nuclei, whereas cytokinesis completes the formation of two separate, independent cells.

   Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

3. 3.

   Outline the role of the cell cycle checkpoints in preventing uncontrolled cell division.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that checkpoints are points in the cell cycle at which the cell checks whether conditions (e.g. DNA integrity, cell size) are suitable to proceed. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that if damaged or incomplete DNA is detected, the cell cycle is halted at a checkpoint to allow repair, or the cell is directed towards apoptosis. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that this control prevents cells with damaged DNA (e.g. mutations) from continuing to divide, which is one mechanism protecting against uncontrolled division such as cancer. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Loss of checkpoint control is a key link between cell cycle biology and cancer — a common extended-response theme.

   Marking points

   • States that checkpoints are points in the cell cycle at which the cell checks whether conditions (e.g. DNA integrity, cell size) are suitable to proceed.
   • States that if damaged or incomplete DNA is detected, the cell cycle is halted at a checkpoint to allow repair, or the cell is directed towards apoptosis.
   • States that this control prevents cells with damaged DNA (e.g. mutations) from continuing to divide, which is one mechanism protecting against uncontrolled division such as cancer.

   Examiner tip: Loss of checkpoint control is a key link between cell cycle biology and cancer — a common extended-response theme.

4. 4.

   Marking analysis: A learner attempts the following task: “Outline the role of the cell cycle checkpoints in preventing uncontrolled cell division.” Their response addresses only this point: “States that checkpoints are points in the cell cycle at which the cell checks whether conditions (e.g. DNA integrity, cell size) are suitable to proceed.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
   2. Requirement 1: Recognises credit for the stated point: States that checkpoints are points in the cell cycle at which the cell checks whether conditions (e.g. DNA integrity, cell size) are suitable to proceed. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   3. Requirement 2: Identifies the missing requirement: States that if damaged or incomplete DNA is detected, the cell cycle is halted at a checkpoint to allow repair, or the cell is directed towards apoptosis. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   4. Requirement 3: Identifies the missing requirement: States that this control prevents cells with damaged DNA (e.g. mutations) from continuing to divide, which is one mechanism protecting against uncontrolled division such as cancer. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

   Marking points

   • Recognises credit for the stated point: States that checkpoints are points in the cell cycle at which the cell checks whether conditions (e.g. DNA integrity, cell size) are suitable to proceed.
   • Identifies the missing requirement: States that if damaged or incomplete DNA is detected, the cell cycle is halted at a checkpoint to allow repair, or the cell is directed towards apoptosis.
   • Identifies the missing requirement: States that this control prevents cells with damaged DNA (e.g. mutations) from continuing to divide, which is one mechanism protecting against uncontrolled division such as cancer.

   Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

5. 5.

   Outline the semi-conservative model of DNA replication.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that the two strands of the parent DNA molecule separate (helicase unwinds the double helix). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that each original (parent) strand acts as a template for the synthesis of a new complementary strand, using free nucleotides and complementary base pairing. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that each resulting DNA molecule consists of one original (conserved) strand and one newly synthesized strand — hence 'semi-conservative'. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The Meselson-Stahl experiment (using density-labelled nitrogen isotopes) is the classic evidence confirming the semi-conservative model over conservative or dispersive alternatives.

   Marking points

   • States that the two strands of the parent DNA molecule separate (helicase unwinds the double helix).
   • States that each original (parent) strand acts as a template for the synthesis of a new complementary strand, using free nucleotides and complementary base pairing.
   • States that each resulting DNA molecule consists of one original (conserved) strand and one newly synthesized strand — hence 'semi-conservative'.

   Examiner tip: The Meselson-Stahl experiment (using density-labelled nitrogen isotopes) is the classic evidence confirming the semi-conservative model over conservative or dispersive alternatives.

6. 6.

   Marking analysis: A learner attempts the following task: “Outline the semi-conservative model of DNA replication.” Their response addresses only this point: “States that the two strands of the parent DNA molecule separate (helicase unwinds the double helix).” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
   2. Requirement 1: Recognises credit for the stated point: States that the two strands of the parent DNA molecule separate (helicase unwinds the double helix). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   3. Requirement 2: Identifies the missing requirement: States that each original (parent) strand acts as a template for the synthesis of a new complementary strand, using free nucleotides and complementary base pairing. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   4. Requirement 3: Identifies the missing requirement: States that each resulting DNA molecule consists of one original (conserved) strand and one newly synthesized strand — hence 'semi-conservative'. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

   Marking points

   • Recognises credit for the stated point: States that the two strands of the parent DNA molecule separate (helicase unwinds the double helix).
   • Identifies the missing requirement: States that each original (parent) strand acts as a template for the synthesis of a new complementary strand, using free nucleotides and complementary base pairing.
   • Identifies the missing requirement: States that each resulting DNA molecule consists of one original (conserved) strand and one newly synthesized strand — hence 'semi-conservative'.

   Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

7. 7.

   Outline the roles of RNA polymerase and complementary base pairing during transcription.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that RNA polymerase binds to DNA and unwinds a section of the double helix at the gene to be transcribed. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that RNA polymerase moves along the template strand, synthesizing a complementary strand of messenger RNA. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that free RNA nucleotides pair with their complementary bases on the template DNA strand (with uracil replacing thymine), and are joined together by RNA polymerase. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Remember that RNA contains uracil instead of thymine, so adenine on the DNA template pairs with uracil, not thymine, in the newly formed mRNA.

   Marking points

   • States that RNA polymerase binds to DNA and unwinds a section of the double helix at the gene to be transcribed.
   • States that RNA polymerase moves along the template strand, synthesizing a complementary strand of messenger RNA.
   • States that free RNA nucleotides pair with their complementary bases on the template DNA strand (with uracil replacing thymine), and are joined together by RNA polymerase.

   Examiner tip: Remember that RNA contains uracil instead of thymine, so adenine on the DNA template pairs with uracil, not thymine, in the newly formed mRNA.

8. 8.

   Marking analysis: A learner attempts the following task: “Outline the roles of RNA polymerase and complementary base pairing during transcription.” Their response addresses only this point: “States that RNA polymerase binds to DNA and unwinds a section of the double helix at the gene to be transcribed.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
   2. Requirement 1: Recognises credit for the stated point: States that RNA polymerase binds to DNA and unwinds a section of the double helix at the gene to be transcribed. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   3. Requirement 2: Identifies the missing requirement: States that RNA polymerase moves along the template strand, synthesizing a complementary strand of messenger RNA. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   4. Requirement 3: Identifies the missing requirement: States that free RNA nucleotides pair with their complementary bases on the template DNA strand (with uracil replacing thymine), and are joined together by RNA polymerase. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

   Marking points

   • Recognises credit for the stated point: States that RNA polymerase binds to DNA and unwinds a section of the double helix at the gene to be transcribed.
   • Identifies the missing requirement: States that RNA polymerase moves along the template strand, synthesizing a complementary strand of messenger RNA.
   • Identifies the missing requirement: States that free RNA nucleotides pair with their complementary bases on the template DNA strand (with uracil replacing thymine), and are joined together by RNA polymerase.

   Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

9. 9.

   Outline the process of translation at the ribosome, including the roles of mRNA, tRNA and codons.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that mRNA binds to a ribosome, and is read in sequential three-base codons. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that tRNA molecules, each carrying a specific amino acid and a complementary anticodon, bind to the mRNA codon by complementary base pairing. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that a peptide bond forms between the amino acid on the incoming tRNA and the growing polypeptide chain. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Develop this part of the answer: States that the ribosome moves along the mRNA to the next codon, and the process repeats until a stop codon is reached, ending translation. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Each codon (three mRNA bases) specifies one amino acid; the genetic code is described as degenerate because most amino acids are specified by more than one codon.

   Marking points

   • States that mRNA binds to a ribosome, and is read in sequential three-base codons.
   • States that tRNA molecules, each carrying a specific amino acid and a complementary anticodon, bind to the mRNA codon by complementary base pairing.
   • States that a peptide bond forms between the amino acid on the incoming tRNA and the growing polypeptide chain.
   • States that the ribosome moves along the mRNA to the next codon, and the process repeats until a stop codon is reached, ending translation.

   Examiner tip: Each codon (three mRNA bases) specifies one amino acid; the genetic code is described as degenerate because most amino acids are specified by more than one codon.

10. 10.

    Marking analysis: A learner attempts the following task: “Outline the process of translation at the ribosome, including the roles of mRNA, tRNA and codons.” Their response addresses only this point: “States that mRNA binds to a ribosome, and is read in sequential three-base codons.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that mRNA binds to a ribosome, and is read in sequential three-base codons. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that tRNA molecules, each carrying a specific amino acid and a complementary anticodon, bind to the mRNA codon by complementary base pairing. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that a peptide bond forms between the amino acid on the incoming tRNA and the growing polypeptide chain. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States that the ribosome moves along the mRNA to the next codon, and the process repeats until a stop codon is reached, ending translation. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that mRNA binds to a ribosome, and is read in sequential three-base codons.
    • Identifies the missing requirement: States that tRNA molecules, each carrying a specific amino acid and a complementary anticodon, bind to the mRNA codon by complementary base pairing.
    • Identifies the missing requirement: States that a peptide bond forms between the amino acid on the incoming tRNA and the growing polypeptide chain.
    • Identifies the missing requirement: States that the ribosome moves along the mRNA to the next codon, and the process repeats until a stop codon is reached, ending translation.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

11. 11.

    In a monohybrid cross, two heterozygous parents Bb are crossed. State the expected genotype ratio and phenotype ratio when B is completely dominant over b.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: Identifies gametes B and b from each parent. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: Obtains offspring BB, Bb, Bb and bb. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States genotype ratio 1 BB : 2 Bb : 1 bb. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States phenotype ratio 3 dominant : 1 recessive. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Keep genotype and phenotype ratios separate; they are not interchangeable.

    Marking points

    • Identifies gametes B and b from each parent.
    • Obtains offspring BB, Bb, Bb and bb.
    • States genotype ratio 1 BB : 2 Bb : 1 bb.
    • States phenotype ratio 3 dominant : 1 recessive.

    Examiner tip: Keep genotype and phenotype ratios separate; they are not interchangeable.

12. 12.

    Marking analysis: A learner attempts the following task: “In a monohybrid cross, two heterozygous parents Bb are crossed. State the expected genotype ratio and phenotype ratio when B is completely dominant over b.” Their response addresses only this point: “Identifies gametes B and b from each parent.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: Identifies gametes B and b from each parent. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: Obtains offspring BB, Bb, Bb and bb. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States genotype ratio 1 BB : 2 Bb : 1 bb. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States phenotype ratio 3 dominant : 1 recessive. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: Identifies gametes B and b from each parent.
    • Identifies the missing requirement: Obtains offspring BB, Bb, Bb and bb.
    • Identifies the missing requirement: States genotype ratio 1 BB : 2 Bb : 1 bb.
    • Identifies the missing requirement: States phenotype ratio 3 dominant : 1 recessive.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

13. 13.

    A DNA template strand has the sequence 3′–TAC GGA CTT–5′. Write the complementary mRNA sequence produced during transcription and state its direction.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: Uses complementary base pairing with U opposite A in RNA. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: Obtains the sequence AUG CCU GAA. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: Writes the direction as 5′–AUG CCU GAA–3′. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: mRNA is written 5′ to 3′ and contains uracil, not thymine.

    Marking points

    • Uses complementary base pairing with U opposite A in RNA.
    • Obtains the sequence AUG CCU GAA.
    • Writes the direction as 5′–AUG CCU GAA–3′.

    Examiner tip: mRNA is written 5′ to 3′ and contains uracil, not thymine.

14. 14.

    Marking analysis: A learner attempts the following task: “A DNA template strand has the sequence 3′–TAC GGA CTT–5′. Write the complementary mRNA sequence produced during transcription and state its direction.” Their response addresses only this point: “Uses complementary base pairing with U opposite A in RNA.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: Uses complementary base pairing with U opposite A in RNA. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: Obtains the sequence AUG CCU GAA. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: Writes the direction as 5′–AUG CCU GAA–3′. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: Uses complementary base pairing with U opposite A in RNA.
    • Identifies the missing requirement: Obtains the sequence AUG CCU GAA.
    • Identifies the missing requirement: Writes the direction as 5′–AUG CCU GAA–3′.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

15. 15.

    Define the terms genotype and phenotype, and outline the relationship between them.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: Defines genotype as the genetic make-up (alleles present) of an organism for a particular trait. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: Defines phenotype as the observable physical or biochemical characteristics of an organism, resulting from its genotype. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that phenotype results from the interaction between genotype and the environment. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Two organisms with different genotypes can share the same phenotype (e.g. BB and Bb both show the dominant phenotype), so genotype and phenotype are not interchangeable terms.

    Marking points

    • Defines genotype as the genetic make-up (alleles present) of an organism for a particular trait.
    • Defines phenotype as the observable physical or biochemical characteristics of an organism, resulting from its genotype.
    • States that phenotype results from the interaction between genotype and the environment.

    Examiner tip: Two organisms with different genotypes can share the same phenotype (e.g. BB and Bb both show the dominant phenotype), so genotype and phenotype are not interchangeable terms.

16. 16.

    Marking analysis: A learner attempts the following task: “Define the terms genotype and phenotype, and outline the relationship between them.” Their response addresses only this point: “Defines genotype as the genetic make-up (alleles present) of an organism for a particular trait.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: Defines genotype as the genetic make-up (alleles present) of an organism for a particular trait. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: Defines phenotype as the observable physical or biochemical characteristics of an organism, resulting from its genotype. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that phenotype results from the interaction between genotype and the environment. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: Defines genotype as the genetic make-up (alleles present) of an organism for a particular trait.
    • Identifies the missing requirement: Defines phenotype as the observable physical or biochemical characteristics of an organism, resulting from its genotype.
    • Identifies the missing requirement: States that phenotype results from the interaction between genotype and the environment.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

17. 17.

    Explain, using an example, how the sex chromosomes determine biological sex in humans.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that humans have 23 pairs of chromosomes, one pair of which are the sex chromosomes (X and Y). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: Explains that since females only produce eggs carrying X, and males produce sperm carrying either X or Y, the sex of the offspring is determined by which type of sperm fertilizes the egg. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Sex determination in humans depends on the sperm, not the egg, since every egg carries an X chromosome while sperm carry either X or Y with equal probability.

    Marking points

    • States that humans have 23 pairs of chromosomes, one pair of which are the sex chromosomes (X and Y).
    • States that females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).
    • Explains that since females only produce eggs carrying X, and males produce sperm carrying either X or Y, the sex of the offspring is determined by which type of sperm fertilizes the egg.

    Examiner tip: Sex determination in humans depends on the sperm, not the egg, since every egg carries an X chromosome while sperm carry either X or Y with equal probability.

18. 18.

    Marking analysis: A learner attempts the following task: “Explain, using an example, how the sex chromosomes determine biological sex in humans.” Their response addresses only this point: “States that humans have 23 pairs of chromosomes, one pair of which are the sex chromosomes (X and Y).” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that humans have 23 pairs of chromosomes, one pair of which are the sex chromosomes (X and Y). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: Explains that since females only produce eggs carrying X, and males produce sperm carrying either X or Y, the sex of the offspring is determined by which type of sperm fertilizes the egg. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that humans have 23 pairs of chromosomes, one pair of which are the sex chromosomes (X and Y).
    • Identifies the missing requirement: States that females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).
    • Identifies the missing requirement: Explains that since females only produce eggs carrying X, and males produce sperm carrying either X or Y, the sex of the offspring is determined by which type of sperm fertilizes the egg.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

19. 19.

    A woman who is a carrier for a recessive X-linked condition (XᴬXᵃ) has children with a man who does not have the condition (XᴬY). Using a Punnett grid, determine the probability that a son will have the condition.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Develop this part of the answer: Identifies maternal gametes as Xᴬ and Xᵃ, and paternal gametes as Xᴬ and Y. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: Constructs a correct 2×2 Punnett grid with offspring XᴬXᴬ, XᴬXᵃ, XᴬY, XᵃY. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: Identifies that among sons only (XᴬY and XᵃY), half are affected (XᵃY). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States the probability as 1/2 (50%) for a son having the condition. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: For X-linked conditions, sons receive their only X chromosome from their mother, so a carrier mother passes the recessive allele to half of her sons regardless of the father's genotype.

    Marking points

    • Identifies maternal gametes as Xᴬ and Xᵃ, and paternal gametes as Xᴬ and Y.
    • Constructs a correct 2×2 Punnett grid with offspring XᴬXᴬ, XᴬXᵃ, XᴬY, XᵃY.
    • Identifies that among sons only (XᴬY and XᵃY), half are affected (XᵃY).
    • States the probability as 1/2 (50%) for a son having the condition.

    Examiner tip: For X-linked conditions, sons receive their only X chromosome from their mother, so a carrier mother passes the recessive allele to half of her sons regardless of the father's genotype.

20. 20.

    Marking analysis: A learner attempts the following task: “A woman who is a carrier for a recessive X-linked condition (XᴬXᵃ) has children with a man who does not have the condition (XᴬY). Using a Punnett grid, determine the probability that a son will have the condition.” Their response addresses only this point: “Identifies maternal gametes as Xᴬ and Xᵃ, and paternal gametes as Xᴬ and Y.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: Identifies maternal gametes as Xᴬ and Xᵃ, and paternal gametes as Xᴬ and Y. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: Constructs a correct 2×2 Punnett grid with offspring XᴬXᴬ, XᴬXᵃ, XᴬY, XᵃY. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: Identifies that among sons only (XᴬY and XᵃY), half are affected (XᵃY). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States the probability as 1/2 (50%) for a son having the condition. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: Identifies maternal gametes as Xᴬ and Xᵃ, and paternal gametes as Xᴬ and Y.
    • Identifies the missing requirement: Constructs a correct 2×2 Punnett grid with offspring XᴬXᴬ, XᴬXᵃ, XᴬY, XᵃY.
    • Identifies the missing requirement: Identifies that among sons only (XᴬY and XᵃY), half are affected (XᵃY).
    • Identifies the missing requirement: States the probability as 1/2 (50%) for a son having the condition.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

21. 21.

    Outline what is meant by a gene mutation, and state one possible effect of a base substitution mutation on the protein produced.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that a gene mutation is a permanent, random change in the nucleotide sequence of DNA. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Work through this mathematical step: States that a base substitution can change a codon such that a different amino acid is inserted into the polypeptide (missense mutation). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Because the genetic code is degenerate, some base substitutions cause no change to the amino acid sequence at all (silent mutations) — not every substitution has a phenotypic effect.

    Marking points

    • States that a gene mutation is a permanent, random change in the nucleotide sequence of DNA.
    • States that a base substitution can change a codon such that a different amino acid is inserted into the polypeptide (missense mutation).

    Examiner tip: Because the genetic code is degenerate, some base substitutions cause no change to the amino acid sequence at all (silent mutations) — not every substitution has a phenotypic effect.

22. 22.

    Marking analysis: A learner attempts the following task: “Outline what is meant by a gene mutation, and state one possible effect of a base substitution mutation on the protein produced.” Their response addresses only this point: “States that a gene mutation is a permanent, random change in the nucleotide sequence of DNA.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that a gene mutation is a permanent, random change in the nucleotide sequence of DNA. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that a base substitution can change a codon such that a different amino acid is inserted into the polypeptide (missense mutation). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that a gene mutation is a permanent, random change in the nucleotide sequence of DNA.
    • Identifies the missing requirement: States that a base substitution can change a codon such that a different amino acid is inserted into the polypeptide (missense mutation).

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

23. 23.

    Distinguish between a gene and an allele.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that a gene is a heritable factor (a section of DNA) that controls a specific characteristic, occupying a specific locus on a chromosome. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that an allele is one specific version (variant form) of a gene, differing from other alleles of the same gene by one or more bases. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A gene is the general category (e.g. 'the gene for flower colour'); an allele is a specific variant of it (e.g. the 'purple' allele or the 'white' allele).

    Marking points

    • States that a gene is a heritable factor (a section of DNA) that controls a specific characteristic, occupying a specific locus on a chromosome.
    • States that an allele is one specific version (variant form) of a gene, differing from other alleles of the same gene by one or more bases.

    Examiner tip: A gene is the general category (e.g. 'the gene for flower colour'); an allele is a specific variant of it (e.g. the 'purple' allele or the 'white' allele).

24. 24.

    Marking analysis: A learner attempts the following task: “Distinguish between a gene and an allele.” Their response addresses only this point: “States that a gene is a heritable factor (a section of DNA) that controls a specific characteristic, occupying a specific locus on a chromosome.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that a gene is a heritable factor (a section of DNA) that controls a specific characteristic, occupying a specific locus on a chromosome. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that an allele is one specific version (variant form) of a gene, differing from other alleles of the same gene by one or more bases. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that a gene is a heritable factor (a section of DNA) that controls a specific characteristic, occupying a specific locus on a chromosome.
    • Identifies the missing requirement: States that an allele is one specific version (variant form) of a gene, differing from other alleles of the same gene by one or more bases.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

25. 25.

    Explain why the number of possible genetically different gametes an organism can produce increases with the number of heterozygous gene pairs, using an organism heterozygous at two independently assorting loci as an example.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that during meiosis, independently assorting gene pairs on different chromosomes segregate independently of one another into gametes. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Work through this mathematical step: States that an organism heterozygous at two independent loci (e.g. AaBb) can produce 2² = 4 genetically distinct gamete types (AB, Ab, aB, ab). Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Develop this part of the answer: States that the number of possible gamete combinations is 2ⁿ, where n is the number of heterozygous gene pairs assorting independently, so more heterozygous pairs produce more genetic variation in gametes. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Independent assortment is a key source of genetic variation distinct from crossing over; the 2ⁿ rule applies only to genes on different chromosomes (or far apart, unlinked, on the same one).

    Marking points

    • States that during meiosis, independently assorting gene pairs on different chromosomes segregate independently of one another into gametes.
    • States that an organism heterozygous at two independent loci (e.g. AaBb) can produce 2² = 4 genetically distinct gamete types (AB, Ab, aB, ab).
    • States that the number of possible gamete combinations is 2ⁿ, where n is the number of heterozygous gene pairs assorting independently, so more heterozygous pairs produce more genetic variation in gametes.

    Examiner tip: Independent assortment is a key source of genetic variation distinct from crossing over; the 2ⁿ rule applies only to genes on different chromosomes (or far apart, unlinked, on the same one).

26. 26.

    Marking analysis: A learner attempts the following task: “Explain why the number of possible genetically different gametes an organism can produce increases with the number of heterozygous gene pairs, using an organism heterozygous at two independently assorting loci as an example.” Their response addresses only this point: “States that during meiosis, independently assorting gene pairs on different chromosomes segregate independently of one another into gametes.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that during meiosis, independently assorting gene pairs on different chromosomes segregate independently of one another into gametes. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that an organism heterozygous at two independent loci (e.g. AaBb) can produce 2² = 4 genetically distinct gamete types (AB, Ab, aB, ab). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that the number of possible gamete combinations is 2ⁿ, where n is the number of heterozygous gene pairs assorting independently, so more heterozygous pairs produce more genetic variation in gametes. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that during meiosis, independently assorting gene pairs on different chromosomes segregate independently of one another into gametes.
    • Identifies the missing requirement: States that an organism heterozygous at two independent loci (e.g. AaBb) can produce 2² = 4 genetically distinct gamete types (AB, Ab, aB, ab).
    • Identifies the missing requirement: States that the number of possible gamete combinations is 2ⁿ, where n is the number of heterozygous gene pairs assorting independently, so more heterozygous pairs produce more genetic variation in gametes.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

27. 27.

    Cystic fibrosis is an autosomal recessive condition. Two unaffected parents, both carriers, have a child. Calculate the probability that the child is unaffected but a carrier.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Develop this part of the answer: Sets up the cross Ff × Ff, with F dominant (unaffected) and f recessive (affected). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: Constructs the resulting genotype ratio 1 FF : 2 Ff : 1 ff. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: Identifies that unaffected carriers are Ff, occurring with probability 2/4, and states the probability as 1/2 (50%). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: 'Unaffected but a carrier' specifically means heterozygous (Ff) — do not simply calculate the probability of being unaffected overall (FF or Ff, which would be 3/4).

    Marking points

    • Sets up the cross Ff × Ff, with F dominant (unaffected) and f recessive (affected).
    • Constructs the resulting genotype ratio 1 FF : 2 Ff : 1 ff.
    • Identifies that unaffected carriers are Ff, occurring with probability 2/4, and states the probability as 1/2 (50%).

    Examiner tip: 'Unaffected but a carrier' specifically means heterozygous (Ff) — do not simply calculate the probability of being unaffected overall (FF or Ff, which would be 3/4).

28. 28.

    Marking analysis: A learner attempts the following task: “Cystic fibrosis is an autosomal recessive condition. Two unaffected parents, both carriers, have a child. Calculate the probability that the child is unaffected but a carrier.” Their response addresses only this point: “Sets up the cross Ff × Ff, with F dominant (unaffected) and f recessive (affected).” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: Sets up the cross Ff × Ff, with F dominant (unaffected) and f recessive (affected). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: Constructs the resulting genotype ratio 1 FF : 2 Ff : 1 ff. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: Identifies that unaffected carriers are Ff, occurring with probability 2/4, and states the probability as 1/2 (50%). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: Sets up the cross Ff × Ff, with F dominant (unaffected) and f recessive (affected).
    • Identifies the missing requirement: Constructs the resulting genotype ratio 1 FF : 2 Ff : 1 ff.
    • Identifies the missing requirement: Identifies that unaffected carriers are Ff, occurring with probability 2/4, and states the probability as 1/2 (50%).

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

29. 29.

    Outline the process of polymerase chain reaction (PCR) and state one application of this technique.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that the DNA sample is heated to separate (denature) the double strands, then cooled to allow short primers to bind to the target sequence. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that a heat-stable DNA polymerase (e.g. Taq polymerase) then extends the primers, synthesizing new complementary strands, and this cycle is repeated many times to exponentially amplify the target DNA sequence. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States a valid application, e.g. forensic DNA profiling, diagnosing genetic or infectious disease, or amplifying DNA for further analysis. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: PCR requires only a tiny initial DNA sample, since each thermal cycle roughly doubles the amount of target DNA present — a defining strength of the technique.

    Marking points

    • States that the DNA sample is heated to separate (denature) the double strands, then cooled to allow short primers to bind to the target sequence.
    • States that a heat-stable DNA polymerase (e.g. Taq polymerase) then extends the primers, synthesizing new complementary strands, and this cycle is repeated many times to exponentially amplify the target DNA sequence.
    • States a valid application, e.g. forensic DNA profiling, diagnosing genetic or infectious disease, or amplifying DNA for further analysis.

    Examiner tip: PCR requires only a tiny initial DNA sample, since each thermal cycle roughly doubles the amount of target DNA present — a defining strength of the technique.

30. 30.

    Marking analysis: A learner attempts the following task: “Outline the process of polymerase chain reaction (PCR) and state one application of this technique.” Their response addresses only this point: “States that the DNA sample is heated to separate (denature) the double strands, then cooled to allow short primers to bind to the target sequence.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that the DNA sample is heated to separate (denature) the double strands, then cooled to allow short primers to bind to the target sequence. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that a heat-stable DNA polymerase (e.g. Taq polymerase) then extends the primers, synthesizing new complementary strands, and this cycle is repeated many times to exponentially amplify the target DNA sequence. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States a valid application, e.g. forensic DNA profiling, diagnosing genetic or infectious disease, or amplifying DNA for further analysis. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that the DNA sample is heated to separate (denature) the double strands, then cooled to allow short primers to bind to the target sequence.
    • Identifies the missing requirement: States that a heat-stable DNA polymerase (e.g. Taq polymerase) then extends the primers, synthesizing new complementary strands, and this cycle is repeated many times to exponentially amplify the target DNA sequence.
    • Identifies the missing requirement: States a valid application, e.g. forensic DNA profiling, diagnosing genetic or infectious disease, or amplifying DNA for further analysis.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

31. 31.

    Explain the mechanism by which increased atmospheric carbon dioxide concentration contributes to the enhanced greenhouse effect and global warming.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that short-wavelength solar radiation passes through the atmosphere and is absorbed at the Earth's surface, which then re-emits energy as longer-wavelength infrared radiation. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that greenhouse gases, including CO₂, absorb this outgoing infrared radiation rather than allowing it to escape directly to space. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that this absorbed energy is re-radiated in all directions, including back towards the Earth's surface, warming the lower atmosphere. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that increasing the concentration of CO₂ increases the amount of infrared radiation absorbed and re-radiated, enhancing this warming effect. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The greenhouse effect itself is natural and necessary for life; it is the enhancement of this effect by rising greenhouse gas concentrations from human activity that drives additional global warming.

    Marking points

    • States that short-wavelength solar radiation passes through the atmosphere and is absorbed at the Earth's surface, which then re-emits energy as longer-wavelength infrared radiation.
    • States that greenhouse gases, including CO₂, absorb this outgoing infrared radiation rather than allowing it to escape directly to space.
    • States that this absorbed energy is re-radiated in all directions, including back towards the Earth's surface, warming the lower atmosphere.
    • States that increasing the concentration of CO₂ increases the amount of infrared radiation absorbed and re-radiated, enhancing this warming effect.

    Examiner tip: The greenhouse effect itself is natural and necessary for life; it is the enhancement of this effect by rising greenhouse gas concentrations from human activity that drives additional global warming.

32. 32.

    Marking analysis: A learner attempts the following task: “Explain the mechanism by which increased atmospheric carbon dioxide concentration contributes to the enhanced greenhouse effect and global warming.” Their response addresses only this point: “States that short-wavelength solar radiation passes through the atmosphere and is absorbed at the Earth's surface, which then re-emits energy as longer-wavelength infrared radiation.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that short-wavelength solar radiation passes through the atmosphere and is absorbed at the Earth's surface, which then re-emits energy as longer-wavelength infrared radiation. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that greenhouse gases, including CO₂, absorb this outgoing infrared radiation rather than allowing it to escape directly to space. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that this absorbed energy is re-radiated in all directions, including back towards the Earth's surface, warming the lower atmosphere. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States that increasing the concentration of CO₂ increases the amount of infrared radiation absorbed and re-radiated, enhancing this warming effect. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that short-wavelength solar radiation passes through the atmosphere and is absorbed at the Earth's surface, which then re-emits energy as longer-wavelength infrared radiation.
    • Identifies the missing requirement: States that greenhouse gases, including CO₂, absorb this outgoing infrared radiation rather than allowing it to escape directly to space.
    • Identifies the missing requirement: States that this absorbed energy is re-radiated in all directions, including back towards the Earth's surface, warming the lower atmosphere.
    • Identifies the missing requirement: States that increasing the concentration of CO₂ increases the amount of infrared radiation absorbed and re-radiated, enhancing this warming effect.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

33. 33.

    Explain how repeated antibiotic use can increase the frequency of antibiotic resistance in a bacterial population by natural selection.

    [5 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: Random mutation produces variation in antibiotic resistance. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: The antibiotic creates a selection pressure. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: Susceptible bacteria die while resistant bacteria survive. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: Survivors reproduce and pass resistance alleles to offspring. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Develop this part of the answer: The resistance allele therefore becomes more frequent over generations. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Antibiotics select existing variation; they do not cause purposeful resistance mutations.

    Marking points

    • Random mutation produces variation in antibiotic resistance.
    • The antibiotic creates a selection pressure.
    • Susceptible bacteria die while resistant bacteria survive.
    • Survivors reproduce and pass resistance alleles to offspring.
    • The resistance allele therefore becomes more frequent over generations.

    Examiner tip: Antibiotics select existing variation; they do not cause purposeful resistance mutations.

34. 34.

    Marking analysis: A learner attempts the following task: “Explain how repeated antibiotic use can increase the frequency of antibiotic resistance in a bacterial population by natural selection.” Their response addresses only this point: “Random mutation produces variation in antibiotic resistance.” Evaluate the response against the complete 5-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [5 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: Random mutation produces variation in antibiotic resistance. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: The antibiotic creates a selection pressure. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: Susceptible bacteria die while resistant bacteria survive. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: Survivors reproduce and pass resistance alleles to offspring. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Requirement 5: Identifies the missing requirement: The resistance allele therefore becomes more frequent over generations. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: Random mutation produces variation in antibiotic resistance.
    • Identifies the missing requirement: The antibiotic creates a selection pressure.
    • Identifies the missing requirement: Susceptible bacteria die while resistant bacteria survive.
    • Identifies the missing requirement: Survivors reproduce and pass resistance alleles to offspring.
    • Identifies the missing requirement: The resistance allele therefore becomes more frequent over generations.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

35. 35.

    Define natural selection and state the conditions necessary for it to occur.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: Defines natural selection as the differential survival and reproduction of individuals due to heritable variation in traits, resulting in a change in allele frequency in a population over generations. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that heritable variation must exist within the population. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that this variation must lead to differences in survival and/or reproductive success (fitness) in a given environment. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Natural selection acts on existing heritable variation — it does not create new variation itself; mutation is the ultimate source of new alleles.

    Marking points

    • Defines natural selection as the differential survival and reproduction of individuals due to heritable variation in traits, resulting in a change in allele frequency in a population over generations.
    • States that heritable variation must exist within the population.
    • States that this variation must lead to differences in survival and/or reproductive success (fitness) in a given environment.

    Examiner tip: Natural selection acts on existing heritable variation — it does not create new variation itself; mutation is the ultimate source of new alleles.

36. 36.

    Marking analysis: A learner attempts the following task: “Define natural selection and state the conditions necessary for it to occur.” Their response addresses only this point: “Defines natural selection as the differential survival and reproduction of individuals due to heritable variation in traits, resulting in a change in allele frequency in a population over generations.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: Defines natural selection as the differential survival and reproduction of individuals due to heritable variation in traits, resulting in a change in allele frequency in a population over generations. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that heritable variation must exist within the population. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that this variation must lead to differences in survival and/or reproductive success (fitness) in a given environment. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: Defines natural selection as the differential survival and reproduction of individuals due to heritable variation in traits, resulting in a change in allele frequency in a population over generations.
    • Identifies the missing requirement: States that heritable variation must exist within the population.
    • Identifies the missing requirement: States that this variation must lead to differences in survival and/or reproductive success (fitness) in a given environment.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

37. 37.

    Outline the roles of insulin and glucagon in regulating blood glucose concentration, including the gland that secretes each hormone and its target tissue.

    [6 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that both hormones are secreted by the pancreas (islets of Langerhans). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that insulin is released when blood glucose is high. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that insulin causes liver and muscle cells to take up glucose and convert it to glycogen. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that this lowers blood glucose concentration toward normal. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Develop this part of the answer: States that glucagon is released when blood glucose is low and acts on the liver to break down glycogen into glucose. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    7. Develop this part of the answer: States that this raises blood glucose concentration toward normal, describing the overall process as negative feedback. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    8. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Name the source gland and target tissue for each hormone explicitly; 'the body' is not a sufficient target.

    Marking points

    • States that both hormones are secreted by the pancreas (islets of Langerhans).
    • States that insulin is released when blood glucose is high.
    • States that insulin causes liver and muscle cells to take up glucose and convert it to glycogen.
    • States that this lowers blood glucose concentration toward normal.
    • States that glucagon is released when blood glucose is low and acts on the liver to break down glycogen into glucose.
    • States that this raises blood glucose concentration toward normal, describing the overall process as negative feedback.

    Examiner tip: Name the source gland and target tissue for each hormone explicitly; 'the body' is not a sufficient target.

38. 38.

    Marking analysis: A learner attempts the following task: “Outline the roles of insulin and glucagon in regulating blood glucose concentration, including the gland that secretes each hormone and its target tissue.” Their response addresses only this point: “States that both hormones are secreted by the pancreas (islets of Langerhans).” Evaluate the response against the complete 6-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [6 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that both hormones are secreted by the pancreas (islets of Langerhans). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that insulin is released when blood glucose is high. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that insulin causes liver and muscle cells to take up glucose and convert it to glycogen. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States that this lowers blood glucose concentration toward normal. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Requirement 5: Identifies the missing requirement: States that glucagon is released when blood glucose is low and acts on the liver to break down glycogen into glucose. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    7. Requirement 6: Identifies the missing requirement: States that this raises blood glucose concentration toward normal, describing the overall process as negative feedback. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    8. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that both hormones are secreted by the pancreas (islets of Langerhans).
    • Identifies the missing requirement: States that insulin is released when blood glucose is high.
    • Identifies the missing requirement: States that insulin causes liver and muscle cells to take up glucose and convert it to glycogen.
    • Identifies the missing requirement: States that this lowers blood glucose concentration toward normal.
    • Identifies the missing requirement: States that glucagon is released when blood glucose is low and acts on the liver to break down glycogen into glucose.
    • Identifies the missing requirement: States that this raises blood glucose concentration toward normal, describing the overall process as negative feedback.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

39. 39.

    Distinguish between the roles of oestrogen and progesterone in regulating the menstrual cycle.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that rising oestrogen, secreted by the developing follicle, stimulates repair and thickening (proliferation) of the uterine lining (endometrium) after menstruation. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that a surge in oestrogen also triggers the luteinizing hormone (LH) surge that causes ovulation. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that progesterone, secreted by the corpus luteum after ovulation, maintains and further develops the uterine lining, preparing it for potential implantation. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that falling progesterone (as the corpus luteum degenerates, if no pregnancy occurs) causes the uterine lining to break down, resulting in menstruation. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Oestrogen dominates the first half of the cycle (follicular phase, building the lining), while progesterone dominates the second half (luteal phase, maintaining the lining) — know which hormone corresponds to which phase.

    Marking points

    • States that rising oestrogen, secreted by the developing follicle, stimulates repair and thickening (proliferation) of the uterine lining (endometrium) after menstruation.
    • States that a surge in oestrogen also triggers the luteinizing hormone (LH) surge that causes ovulation.
    • States that progesterone, secreted by the corpus luteum after ovulation, maintains and further develops the uterine lining, preparing it for potential implantation.
    • States that falling progesterone (as the corpus luteum degenerates, if no pregnancy occurs) causes the uterine lining to break down, resulting in menstruation.

    Examiner tip: Oestrogen dominates the first half of the cycle (follicular phase, building the lining), while progesterone dominates the second half (luteal phase, maintaining the lining) — know which hormone corresponds to which phase.

40. 40.

    Marking analysis: A learner attempts the following task: “Distinguish between the roles of oestrogen and progesterone in regulating the menstrual cycle.” Their response addresses only this point: “States that rising oestrogen, secreted by the developing follicle, stimulates repair and thickening (proliferation) of the uterine lining (endometrium) after menstruation.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that rising oestrogen, secreted by the developing follicle, stimulates repair and thickening (proliferation) of the uterine lining (endometrium) after menstruation. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that a surge in oestrogen also triggers the luteinizing hormone (LH) surge that causes ovulation. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that progesterone, secreted by the corpus luteum after ovulation, maintains and further develops the uterine lining, preparing it for potential implantation. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States that falling progesterone (as the corpus luteum degenerates, if no pregnancy occurs) causes the uterine lining to break down, resulting in menstruation. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that rising oestrogen, secreted by the developing follicle, stimulates repair and thickening (proliferation) of the uterine lining (endometrium) after menstruation.
    • Identifies the missing requirement: States that a surge in oestrogen also triggers the luteinizing hormone (LH) surge that causes ovulation.
    • Identifies the missing requirement: States that progesterone, secreted by the corpus luteum after ovulation, maintains and further develops the uterine lining, preparing it for potential implantation.
    • Identifies the missing requirement: States that falling progesterone (as the corpus luteum degenerates, if no pregnancy occurs) causes the uterine lining to break down, resulting in menstruation.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

41. 41.

    Explain how negative feedback maintains core body temperature when it rises above the normal set point.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that thermoreceptors (e.g. in the hypothalamus and skin) detect that body temperature has risen above the set point. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that the hypothalamus, acting as the control centre, triggers responses including vasodilation of skin blood vessels (increasing heat loss) and increased sweating (heat loss through evaporation). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that these responses lower body temperature back towards the set point, at which point the corrective responses are switched off — this reduction in the original stimulus is what makes it negative feedback. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Negative feedback always works by reversing the direction of the original change — a rise in temperature triggers heat-losing responses, and a fall triggers heat-generating/conserving responses.

    Marking points

    • States that thermoreceptors (e.g. in the hypothalamus and skin) detect that body temperature has risen above the set point.
    • States that the hypothalamus, acting as the control centre, triggers responses including vasodilation of skin blood vessels (increasing heat loss) and increased sweating (heat loss through evaporation).
    • States that these responses lower body temperature back towards the set point, at which point the corrective responses are switched off — this reduction in the original stimulus is what makes it negative feedback.

    Examiner tip: Negative feedback always works by reversing the direction of the original change — a rise in temperature triggers heat-losing responses, and a fall triggers heat-generating/conserving responses.

42. 42.

    Marking analysis: A learner attempts the following task: “Explain how negative feedback maintains core body temperature when it rises above the normal set point.” Their response addresses only this point: “States that thermoreceptors (e.g. in the hypothalamus and skin) detect that body temperature has risen above the set point.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that thermoreceptors (e.g. in the hypothalamus and skin) detect that body temperature has risen above the set point. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that the hypothalamus, acting as the control centre, triggers responses including vasodilation of skin blood vessels (increasing heat loss) and increased sweating (heat loss through evaporation). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that these responses lower body temperature back towards the set point, at which point the corrective responses are switched off — this reduction in the original stimulus is what makes it negative feedback. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that thermoreceptors (e.g. in the hypothalamus and skin) detect that body temperature has risen above the set point.
    • Identifies the missing requirement: States that the hypothalamus, acting as the control centre, triggers responses including vasodilation of skin blood vessels (increasing heat loss) and increased sweating (heat loss through evaporation).
    • Identifies the missing requirement: States that these responses lower body temperature back towards the set point, at which point the corrective responses are switched off — this reduction in the original stimulus is what makes it negative feedback.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.