Biology

1. 1.

   Explain why the rate of an enzyme-controlled reaction increases from 20 °C to 35 °C but decreases sharply above 45 °C.

   [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: Increasing temperature gives particles more kinetic energy. 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: Successful collisions between enzyme and substrate become more frequent. 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: Above the optimum, bonds maintaining the enzyme shape are disrupted. 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: The active site changes shape, so fewer enzyme-substrate complexes form. 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: Do not say the enzyme dies; describe denaturation and the active site.

   Marking points

   • Increasing temperature gives particles more kinetic energy.
   • Successful collisions between enzyme and substrate become more frequent.
   • Above the optimum, bonds maintaining the enzyme shape are disrupted.
   • The active site changes shape, so fewer enzyme-substrate complexes form.

   Examiner tip: Do not say the enzyme dies; describe denaturation and the active site.

2. 2.

   In pea plants, tall T is dominant to short t. Cross a heterozygous tall plant with a short plant and state the expected phenotype ratio.

   [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: Writes the parental genotypes Tt and tt. 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: Identifies gametes T and t from one parent and t from the other. 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: Obtains offspring Tt and tt in equal proportions. 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 1 tall : 1 short. 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: Use the parental genotypes to build the cross before stating the ratio.

   Marking points

   • Writes the parental genotypes Tt and tt.
   • Identifies gametes T and t from one parent and t from the other.
   • Obtains offspring Tt and tt in equal proportions.
   • States phenotype ratio 1 tall : 1 short.

   Examiner tip: Use the parental genotypes to build the cross before stating the ratio.

3. 3.

   Define the term enzyme, and state what happens to the rate of an enzyme-controlled reaction if the substrate concentration is increased while enzyme concentration stays constant, at low substrate concentration.

   [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: Defines an enzyme as a biological catalyst, a protein that speeds up the rate of a specific chemical reaction without being used up. 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 at low substrate concentration, increasing substrate concentration increases the rate of reaction. 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: At low substrate concentration, there are 'spare' active sites available, so more substrate means more enzyme-substrate complexes forming per unit time.

   Marking points

   • Defines an enzyme as a biological catalyst, a protein that speeds up the rate of a specific chemical reaction without being used up.
   • States that at low substrate concentration, increasing substrate concentration increases the rate of reaction.

   Examiner tip: At low substrate concentration, there are 'spare' active sites available, so more substrate means more enzyme-substrate complexes forming per unit time.

4. 4.

   Describe how the 'lock and key' model explains enzyme specificity, referring to the shape of the active site.

   [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 an enzyme has an active site with a specific shape. 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 only a substrate with a complementary shape to the active site can bind (like a key fitting a specific lock). 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 explains why each enzyme is specific and can usually only catalyse one particular reaction (or a small group of very similar reactions). 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 'lock and key' model explains specificity through fixed shape-matching — it is an older, simplified model compared to the more flexible 'induced fit' model taught at higher levels.

   Marking points

   • States that an enzyme has an active site with a specific shape.
   • States that only a substrate with a complementary shape to the active site can bind (like a key fitting a specific lock).
   • States that this explains why each enzyme is specific and can usually only catalyse one particular reaction (or a small group of very similar reactions).

   Examiner tip: The 'lock and key' model explains specificity through fixed shape-matching — it is an older, simplified model compared to the more flexible 'induced fit' model taught at higher levels.

5. 5.

   Distinguish between continuous variation and discontinuous variation, giving one human example of each.

   [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 continuous variation shows a full range of intermediate values between two extremes, e.g. human height. 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 discontinuous variation shows distinct, separate categories with no intermediate values, e.g. human blood group (A, B, AB, O). 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: Continuous variation is usually the result of many genes acting together plus environmental influence, while discontinuous variation is usually controlled by a single gene.

   Marking points

   • States that continuous variation shows a full range of intermediate values between two extremes, e.g. human height.
   • States that discontinuous variation shows distinct, separate categories with no intermediate values, e.g. human blood group (A, B, AB, O).

   Examiner tip: Continuous variation is usually the result of many genes acting together plus environmental influence, while discontinuous variation is usually controlled by a single gene.

6. 6.

   In humans, blood group is controlled by three alleles: IA, IB and Iᴼ, where IA and IB are both dominant to Iᴼ. A person with genotype IAIᴼ is crossed with a person with genotype IBIᴼ. State the possible genotypes of their children.

   [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 IA and Iᴼ from the first parent, and IB and Iᴼ from the second 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: Constructs the combinations correctly using a Punnett grid. 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 the four possible genotypes: IAIB, IAIᴼ, IBIᴼ, IᴼIᴼ. 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 each occurs with equal probability (1/4 each). 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: With three alleles for one gene (multiple alleles), a Punnett grid still works exactly the same way as with two alleles — just list all possible gametes from each parent.

   Marking points

   • Identifies gametes IA and Iᴼ from the first parent, and IB and Iᴼ from the second parent.
   • Constructs the combinations correctly using a Punnett grid.
   • States the four possible genotypes: IAIB, IAIᴼ, IBIᴼ, IᴼIᴼ.
   • States each occurs with equal probability (1/4 each).

   Examiner tip: With three alleles for one gene (multiple alleles), a Punnett grid still works exactly the same way as with two alleles — just list all possible gametes from each parent.

7. 7.

   Explain why immobilising an enzyme (e.g. on beads) is useful in industrial processes, compared to using a free enzyme in solution.

   [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 an immobilised enzyme is fixed in place (e.g. attached to beads or a membrane) rather than dissolved freely in the reaction mixture. 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 this allows the enzyme to be easily separated from the product and reused multiple times, reducing cost. 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 it avoids contaminating the product with the enzyme itself, which is important for products like food or pharmaceuticals. 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: Enzyme immobilisation is widely used industrially (e.g. lactase production of lactose-free milk) specifically because it allows continuous reuse of an expensive biological catalyst.

   Marking points

   • States that an immobilised enzyme is fixed in place (e.g. attached to beads or a membrane) rather than dissolved freely in the reaction mixture.
   • States that this allows the enzyme to be easily separated from the product and reused multiple times, reducing cost.
   • States that it avoids contaminating the product with the enzyme itself, which is important for products like food or pharmaceuticals.

   Examiner tip: Enzyme immobilisation is widely used industrially (e.g. lactase production of lactose-free milk) specifically because it allows continuous reuse of an expensive biological catalyst.

8. 8.

   Outline how a mutation can lead to a new allele of a gene, and state whether all mutations are harmful.

   [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 a mutation is a random change in the DNA base sequence of a gene. 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 this changed base sequence can create a new version (allele) of that gene, which may produce a slightly different protein. 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 not all mutations are harmful — some are neutral (no noticeable effect) and, rarely, some can even be beneficial to the organism. 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: Mutations are the ultimate source of all new alleles and therefore all genetic variation — without mutation, natural selection would have no new variation to act upon.

   Marking points

   • States that a mutation is a random change in the DNA base sequence of a gene.
   • States that this changed base sequence can create a new version (allele) of that gene, which may produce a slightly different protein.
   • States that not all mutations are harmful — some are neutral (no noticeable effect) and, rarely, some can even be beneficial to the organism.

   Examiner tip: Mutations are the ultimate source of all new alleles and therefore all genetic variation — without mutation, natural selection would have no new variation to act upon.

9. 9.

   Explain why enzymes are described as biological catalysts, referring to activation energy.

   [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 a catalyst speeds up the rate of a chemical reaction without being permanently changed or used up itself. 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 enzymes achieve this by lowering the activation energy needed for the reaction to occur. 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 they are described as 'biological' catalysts because they are produced by living organisms and are proteins (unlike inorganic catalysts). 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: An enzyme is chemically unchanged at the end of the reaction it catalyses, which is why a small amount of enzyme can process a very large amount of substrate over time.

   Marking points

   • States that a catalyst speeds up the rate of a chemical reaction without being permanently changed or used up itself.
   • States that enzymes achieve this by lowering the activation energy needed for the reaction to occur.
   • States that they are described as 'biological' catalysts because they are produced by living organisms and are proteins (unlike inorganic catalysts).

   Examiner tip: An enzyme is chemically unchanged at the end of the reaction it catalyses, which is why a small amount of enzyme can process a very large amount of substrate over time.

10. 10.

    A pea plant with round seeds (genotype Rr) is self-pollinated. Round R is dominant to wrinkled r. Determine the expected phenotype ratio of the offspring.

    [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: Identifies gametes R and r from the self-pollinating Rr 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: Constructs the cross to obtain offspring genotypes 1 RR : 2 Rr : 1 rr. 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 the phenotype ratio as 3 round : 1 wrinkled. 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: Self-pollination of a heterozygote (Rr × Rr) always produces the classic 3:1 dominant-to-recessive phenotype ratio for a single gene with complete dominance.

    Marking points

    • Identifies gametes R and r from the self-pollinating Rr parent.
    • Constructs the cross to obtain offspring genotypes 1 RR : 2 Rr : 1 rr.
    • States the phenotype ratio as 3 round : 1 wrinkled.

    Examiner tip: Self-pollination of a heterozygote (Rr × Rr) always produces the classic 3:1 dominant-to-recessive phenotype ratio for a single gene with complete dominance.

11. 11.

    Explain why enzyme activity is low at very low pH and very high pH, but high at the optimum pH.

    [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 each enzyme has an optimum pH at which its rate of reaction is greatest. 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 at pH values far from the optimum (too acidic or too alkaline), the bonds holding the enzyme's tertiary structure together are disrupted. 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 changes (denatures) the shape of the enzyme's active site, so the substrate can no longer bind, and the rate of reaction decreases. 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: pH affects enzyme activity through exactly the same mechanism as extreme temperature — both disrupt the bonds maintaining the enzyme's shape, denaturing its active site.

    Marking points

    • States that each enzyme has an optimum pH at which its rate of reaction is greatest.
    • States that at pH values far from the optimum (too acidic or too alkaline), the bonds holding the enzyme's tertiary structure together are disrupted.
    • States that this changes (denatures) the shape of the enzyme's active site, so the substrate can no longer bind, and the rate of reaction decreases.

    Examiner tip: pH affects enzyme activity through exactly the same mechanism as extreme temperature — both disrupt the bonds maintaining the enzyme's shape, denaturing its active site.

12. 12.

    Describe the structure of DNA, and state what is meant by the term gene.

    [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 DNA is a double helix (two strands wound around each other) made up of repeating units called nucleotides. 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 nucleotide contains one of four bases (A, T, C, G), and complementary base pairing (A with T, C with G) holds the two strands together. 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 gene is a section (length) of DNA that codes for (contains the instructions for) a specific protein. 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: Complementary base pairing (A-T, C-G) is what allows DNA to be copied accurately — each strand acts as a template that determines the exact sequence of its new partner strand.

    Marking points

    • States that DNA is a double helix (two strands wound around each other) made up of repeating units called nucleotides.
    • States that each nucleotide contains one of four bases (A, T, C, G), and complementary base pairing (A with T, C with G) holds the two strands together.
    • States that a gene is a section (length) of DNA that codes for (contains the instructions for) a specific protein.

    Examiner tip: Complementary base pairing (A-T, C-G) is what allows DNA to be copied accurately — each strand acts as a template that determines the exact sequence of its new partner strand.

13. 13.

    State the relationship between chromosomes, genes and alleles.

    [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 a chromosome is a long, thread-like structure made of DNA, found in the nucleus of a cell. 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 gene is a section of a chromosome (DNA) that codes for a particular characteristic/protein. 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 an allele is one of the different (alternative) versions of a particular gene, controlling different versions of the same characteristic. 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: Think of it as a hierarchy: a chromosome is made of many genes, and each gene can exist as different alleles — this hierarchy (chromosome → gene → allele) is worth memorising in order.

    Marking points

    • States that a chromosome is a long, thread-like structure made of DNA, found in the nucleus of a cell.
    • States that a gene is a section of a chromosome (DNA) that codes for a particular characteristic/protein.
    • States that an allele is one of the different (alternative) versions of a particular gene, controlling different versions of the same characteristic.

    Examiner tip: Think of it as a hierarchy: a chromosome is made of many genes, and each gene can exist as different alleles — this hierarchy (chromosome → gene → allele) is worth memorising in order.

14. 14.

    Distinguish between mitosis and meiosis, in terms of the number and genetic content of the cells produced.

    [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 mitosis produces two daughter cells that are genetically identical to the parent cell and to each other, and are diploid (the same chromosome number as the parent cell). 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 mitosis is used for growth, repair of tissues, and asexual reproduction. 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 meiosis produces four daughter cells (gametes) that are genetically different from each other and from the parent cell, and are haploid (half the chromosome number of the parent cell). 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 meiosis is used to produce gametes for sexual reproduction, introducing genetic variation. 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: Mitosis keeps chromosome number constant and produces identical cells (for growth/repair); meiosis halves chromosome number and produces genetically varied cells (for sexual reproduction) — this contrast is the key idea.

    Marking points

    • States that mitosis produces two daughter cells that are genetically identical to the parent cell and to each other, and are diploid (the same chromosome number as the parent cell).
    • States that mitosis is used for growth, repair of tissues, and asexual reproduction.
    • States that meiosis produces four daughter cells (gametes) that are genetically different from each other and from the parent cell, and are haploid (half the chromosome number of the parent cell).
    • States that meiosis is used to produce gametes for sexual reproduction, introducing genetic variation.

    Examiner tip: Mitosis keeps chromosome number constant and produces identical cells (for growth/repair); meiosis halves chromosome number and produces genetically varied cells (for sexual reproduction) — this contrast is the key idea.

15. 15.

    In humans, sex is determined by the X and Y chromosomes: females are XX and males are XY. Using a genetic diagram, explain why approximately half of all offspring are expected to be male and half female.

    [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 all eggs produced by the mother contain a single X 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 sperm produced by the father contain either an X chromosome or a Y chromosome, in equal proportions. 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: Constructs the cross showing the possible offspring genotypes: XX (female) and XY (male). 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 since there is an equal (1:1) chance of an egg being fertilised by an X-carrying or Y-carrying sperm, approximately half of offspring will be female and half will be male. 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: It is always the father's sperm, not the mother's egg, that determines the sex of a child, since every egg carries an X chromosome while sperm can carry either X or Y.

    Marking points

    • States that all eggs produced by the mother contain a single X chromosome.
    • States that sperm produced by the father contain either an X chromosome or a Y chromosome, in equal proportions.
    • Constructs the cross showing the possible offspring genotypes: XX (female) and XY (male).
    • States that since there is an equal (1:1) chance of an egg being fertilised by an X-carrying or Y-carrying sperm, approximately half of offspring will be female and half will be male.

    Examiner tip: It is always the father's sperm, not the mother's egg, that determines the sex of a child, since every egg carries an X chromosome while sperm can carry either X or Y.

16. 16.

    A pea plant with round seeds has an unknown genotype, either RR or Rr (round R is dominant to wrinkled r). Describe how a test cross could be used to determine its genotype.

    [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 round-seeded plant of unknown genotype is crossed with a plant known to be homozygous recessive (genotype rr, wrinkled seeds). 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 all of the offspring have round seeds, the unknown plant's genotype is homozygous dominant, RR. 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 if approximately half of the offspring have round seeds and half have wrinkled seeds (a 1:1 ratio), the unknown plant's genotype is heterozygous, Rr. 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: A test cross always uses a homozygous recessive partner, since this is the only genotype that reveals the unknown parent's hidden alleles directly in the offspring's phenotype ratio.

    Marking points

    • States that the round-seeded plant of unknown genotype is crossed with a plant known to be homozygous recessive (genotype rr, wrinkled seeds).
    • States that if all of the offspring have round seeds, the unknown plant's genotype is homozygous dominant, RR.
    • States that if approximately half of the offspring have round seeds and half have wrinkled seeds (a 1:1 ratio), the unknown plant's genotype is heterozygous, Rr.

    Examiner tip: A test cross always uses a homozygous recessive partner, since this is the only genotype that reveals the unknown parent's hidden alleles directly in the offspring's phenotype ratio.

17. 17.

    In snapdragon plants, flower colour shows codominance: allele CR produces red pigment and allele CW produces white pigment, with heterozygous plants (CRCW) being pink. Explain what is meant by codominance using this example, and determine the phenotype ratio produced when two pink (CRCW) plants are crossed.

    [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: States that codominance occurs when two different alleles of a gene are both fully expressed in the phenotype of a heterozygote, with neither allele being recessive to the other. 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 in the heterozygote CRCW, both red and white pigments are produced, resulting in a pink phenotype, rather than one colour completely masking the other. 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: Constructs the cross CRCW × CRCW to obtain offspring genotypes 1 CRCR : 2 CRCW : 1 CWCW. 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 phenotype ratio: 1 red : 2 pink : 1 white. 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: Codominance is the reason a cross between two heterozygotes can produce three distinct phenotypes instead of the usual two seen with complete dominance — the heterozygote gets its own, visibly intermediate phenotype.

    Marking points

    • States that codominance occurs when two different alleles of a gene are both fully expressed in the phenotype of a heterozygote, with neither allele being recessive to the other.
    • States that in the heterozygote CRCW, both red and white pigments are produced, resulting in a pink phenotype, rather than one colour completely masking the other.
    • Constructs the cross CRCW × CRCW to obtain offspring genotypes 1 CRCR : 2 CRCW : 1 CWCW.
    • States the phenotype ratio: 1 red : 2 pink : 1 white.

    Examiner tip: Codominance is the reason a cross between two heterozygotes can produce three distinct phenotypes instead of the usual two seen with complete dominance — the heterozygote gets its own, visibly intermediate phenotype.

18. 18.

    Explain, using the example of antibiotic resistance in bacteria, how natural selection can lead to a change in a population over time.

    [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 within a bacterial population, there is natural (random) genetic variation, so a few individual bacteria may already carry a mutation giving some resistance to an antibiotic. 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 when the antibiotic is used, non-resistant bacteria are killed, while the resistant bacteria survive. 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 the surviving resistant bacteria reproduce, passing the resistance allele on to their offspring. 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 over many generations, the proportion of resistant bacteria in the population increases, since resistant individuals consistently have greater reproductive success. 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: Antibiotics do not cause the resistance mutation to appear — the resistant bacteria already exist by chance beforehand; the antibiotic simply provides the selection pressure that favours their survival and reproduction.

    Marking points

    • States that within a bacterial population, there is natural (random) genetic variation, so a few individual bacteria may already carry a mutation giving some resistance to an antibiotic.
    • States that when the antibiotic is used, non-resistant bacteria are killed, while the resistant bacteria survive.
    • States that the surviving resistant bacteria reproduce, passing the resistance allele on to their offspring.
    • States that over many generations, the proportion of resistant bacteria in the population increases, since resistant individuals consistently have greater reproductive success.

    Examiner tip: Antibiotics do not cause the resistance mutation to appear — the resistant bacteria already exist by chance beforehand; the antibiotic simply provides the selection pressure that favours their survival and reproduction.

19. 19.

    Distinguish between natural selection and selective breeding (artificial selection), giving an example of selective breeding.

    [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 in natural selection, the environment determines which individuals survive and reproduce, based on naturally occurring variation. 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 in selective breeding, humans (not the natural environment) choose which individuals with desirable characteristics are bred together. 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: Gives a valid example, e.g. breeding cows that produce the highest milk yield together over many generations to increase average milk yield in the herd. 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 underlying mechanism (individuals with certain alleles being more likely to reproduce) is identical in both processes — the only real difference is who or what is doing the selecting.

    Marking points

    • States that in natural selection, the environment determines which individuals survive and reproduce, based on naturally occurring variation.
    • States that in selective breeding, humans (not the natural environment) choose which individuals with desirable characteristics are bred together.
    • Gives a valid example, e.g. breeding cows that produce the highest milk yield together over many generations to increase average milk yield in the herd.

    Examiner tip: The underlying mechanism (individuals with certain alleles being more likely to reproduce) is identical in both processes — the only real difference is who or what is doing the selecting.

20. 20.

    Outline the basic process of genetic engineering used to produce human insulin using bacteria.

    [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 the gene for human insulin is identified and cut out of human DNA (a chromosome), using an enzyme. 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 this gene is inserted into a bacterial plasmid (a small circular piece of DNA). 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 the plasmid, now containing the human insulin gene, is inserted into a bacterium. 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 genetically modified bacterium is grown (cultured) in large fermenters, where it expresses the gene and produces human insulin, which is then extracted and purified. 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: Plasmids are the key tool that makes genetic engineering possible — they act as a vector, carrying a foreign gene into a bacterium's cell where it can then be expressed.

    Marking points

    • States that the gene for human insulin is identified and cut out of human DNA (a chromosome), using an enzyme.
    • States that this gene is inserted into a bacterial plasmid (a small circular piece of DNA).
    • States that the plasmid, now containing the human insulin gene, is inserted into a bacterium.
    • States that the genetically modified bacterium is grown (cultured) in large fermenters, where it expresses the gene and produces human insulin, which is then extracted and purified.

    Examiner tip: Plasmids are the key tool that makes genetic engineering possible — they act as a vector, carrying a foreign gene into a bacterium's cell where it can then be expressed.

21. 21.

    In an experiment, substrate concentration is kept very high while enzyme concentration is varied. State and explain the effect on the rate of reaction as enzyme concentration is increased.

    [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 increasing enzyme concentration increases the rate of reaction. 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: Explains this is because there are more active sites available for the substrate to bind to, so more enzyme-substrate complexes can form per unit time. 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 eventually, if substrate concentration becomes limiting instead, further increases in enzyme concentration will no longer increase the rate of reaction. 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: Whichever factor (enzyme or substrate concentration) is currently in short supply becomes the 'limiting factor' — increasing anything else has no further effect until that limiting factor itself is increased.

    Marking points

    • States that increasing enzyme concentration increases the rate of reaction.
    • Explains this is because there are more active sites available for the substrate to bind to, so more enzyme-substrate complexes can form per unit time.
    • States that eventually, if substrate concentration becomes limiting instead, further increases in enzyme concentration will no longer increase the rate of reaction.

    Examiner tip: Whichever factor (enzyme or substrate concentration) is currently in short supply becomes the 'limiting factor' — increasing anything else has no further effect until that limiting factor itself is increased.

22. 22.

    Describe an experiment to investigate the effect of temperature on the rate of activity of the enzyme amylase, using iodine solution to test for starch.

    [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 a mixture of starch solution and amylase is incubated at a chosen temperature, using a water bath. 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 at regular time intervals, a sample of the mixture is removed and tested with iodine solution, which turns blue-black in the presence of starch. 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 the time taken for the iodine solution to no longer turn blue-black (staying orange/brown) is recorded, indicating that all the starch has been broken down. 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 procedure is repeated at a range of different temperatures, and the times compared to determine the effect of temperature on the rate of enzyme activity. 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: A shorter time for the iodine test to stop turning blue-black indicates a faster rate of starch breakdown by amylase — time taken is being used here as an inverse measure of reaction rate.

    Marking points

    • States that a mixture of starch solution and amylase is incubated at a chosen temperature, using a water bath.
    • States that at regular time intervals, a sample of the mixture is removed and tested with iodine solution, which turns blue-black in the presence of starch.
    • States that the time taken for the iodine solution to no longer turn blue-black (staying orange/brown) is recorded, indicating that all the starch has been broken down.
    • States that this procedure is repeated at a range of different temperatures, and the times compared to determine the effect of temperature on the rate of enzyme activity.

    Examiner tip: A shorter time for the iodine test to stop turning blue-black indicates a faster rate of starch breakdown by amylase — time taken is being used here as an inverse measure of reaction rate.

23. 23.

    Red-green colour blindness in humans is caused by a recessive allele carried on the X chromosome. A woman who is a carrier for colour blindness (genotype XᴺXⁿ) has children with a man with normal vision (genotype XᴺY). Determine the possible genotypes and phenotypes of their children, and state the probability that a son will be colour blind.

    [5 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 gametes from the mother as Xᴺ and Xⁿ, and from the father as Xᴺ 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 the cross to obtain the four possible offspring genotypes: XᴺXᴺ, XᴺXⁿ, XᴺY, XⁿY. 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 daughters (XᴺXᴺ or XᴺXⁿ) will all have normal vision, since they always inherit a normal allele from their father. 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 sons have a 1/2 (50%) chance of being XᴺY (normal vision) and a 1/2 (50%) chance of being XⁿY (colour blind), since sons only inherit their single X chromosome from their mother. 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 the overall probability that a son will be colour blind as 1/2 (50%). 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: For a sex-linked recessive condition, daughters are almost never affected when the father has normal vision, since they always receive one normal allele from him — this asymmetry between sons and daughters is the hallmark of sex-linked inheritance.

    Marking points

    • Identifies gametes from the mother as Xᴺ and Xⁿ, and from the father as Xᴺ and Y.
    • Constructs the cross to obtain the four possible offspring genotypes: XᴺXᴺ, XᴺXⁿ, XᴺY, XⁿY.
    • States that daughters (XᴺXᴺ or XᴺXⁿ) will all have normal vision, since they always inherit a normal allele from their father.
    • States that sons have a 1/2 (50%) chance of being XᴺY (normal vision) and a 1/2 (50%) chance of being XⁿY (colour blind), since sons only inherit their single X chromosome from their mother.
    • States the overall probability that a son will be colour blind as 1/2 (50%).

    Examiner tip: For a sex-linked recessive condition, daughters are almost never affected when the father has normal vision, since they always receive one normal allele from him — this asymmetry between sons and daughters is the hallmark of sex-linked inheritance.

24. 24.

    State two possible causes of variation between individuals of the same species, and explain how each contributes to variation.

    [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 a genetic cause, e.g. mutation and the shuffling/combination of alleles during meiosis and sexual reproduction. 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: Explains that these processes produce offspring with new or different combinations of alleles compared with their parents. 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 an environmental cause, e.g. differences in diet, exercise, climate or other external conditions experienced by an individual. 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: Explains that these environmental factors can affect an individual's characteristics (phenotype) without changing its genes, e.g. identical (genetically the same) twins can end up with different heights or weights due to differences in their diet. 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: Identical twins are the clearest real-world evidence that environment alone can cause noticeable variation, since any differences between them cannot be explained by genetics at all.

    Marking points

    • States a genetic cause, e.g. mutation and the shuffling/combination of alleles during meiosis and sexual reproduction.
    • Explains that these processes produce offspring with new or different combinations of alleles compared with their parents.
    • States an environmental cause, e.g. differences in diet, exercise, climate or other external conditions experienced by an individual.
    • Explains that these environmental factors can affect an individual's characteristics (phenotype) without changing its genes, e.g. identical (genetically the same) twins can end up with different heights or weights due to differences in their diet.

    Examiner tip: Identical twins are the clearest real-world evidence that environment alone can cause noticeable variation, since any differences between them cannot be explained by genetics at all.

25. 25.

    Enzymes such as proteases and lipases are added to biological washing powders. Explain why these enzymes are effective at removing certain types of stains from clothing at relatively low washing temperatures.

    [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 proteases break down protein-based stains, e.g. blood or egg, into smaller, soluble molecules that can be washed away. 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 lipases break down fat/grease-based stains into smaller, soluble molecules that can be washed away. 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 because enzymes catalyse these breakdown reactions, effective stain removal can occur at lower washing temperatures than would otherwise be needed, saving energy. 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: Biological washing powders should not be used at very high temperatures, since this would denature the enzymes and destroy their stain-removing effectiveness — the whole point is that they work well at lower, energy-saving temperatures.

    Marking points

    • States that proteases break down protein-based stains, e.g. blood or egg, into smaller, soluble molecules that can be washed away.
    • States that lipases break down fat/grease-based stains into smaller, soluble molecules that can be washed away.
    • States that because enzymes catalyse these breakdown reactions, effective stain removal can occur at lower washing temperatures than would otherwise be needed, saving energy.

    Examiner tip: Biological washing powders should not be used at very high temperatures, since this would denature the enzymes and destroy their stain-removing effectiveness — the whole point is that they work well at lower, energy-saving temperatures.

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