AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2024: Question 3

✅ Correct answer

The allele is recessive because parents who do not have AKU have children who do have AKU.

For example, A and B do not have AKU, but their child D has AKU. This shows the AKU allele must be recessive.

💡 Key knowledge

• A recessive disorder can appear in children even when the parents do not show the disorder.
• This happens because the parents are likely to be carriers.
• Two recessive alleles are needed for the disorder to be shown.

🧠 Exam technique

• Use the pedigree as evidence.
• State the pattern clearly: unaffected parents → affected child.
• One good example is enough for the mark.

❌ Common errors

• Saying “because some people have AKU” is not enough.
• Talking about dominant alleles instead of using the family evidence.
• Forgetting to mention that the parents do not have AKU.

✅ Correct answer

G

G must be heterozygous because G does not have AKU but has children who do have AKU, so G must carry the recessive allele.

💡 Key knowledge

• For a recessive disorder, an unaffected parent with affected children is often a carrier.
• Heterozygous means one dominant allele and one recessive allele.

🧠 Exam technique

• Look for the person who must pass on the recessive allele.
• “Definitely” means there is only one answer supported by the family tree.

❌ Common errors

• Picking D because D has AKU — affected people are usually homozygous recessive, not heterozygous.
• Choosing C or E without enough evidence.

📐 Calculations: Punnett square step-by-step

1. The female has AKU, so her genotype must be aa.
2. The male is heterozygous, so his genotype is Aa.
3. Write the gametes:
   • Female gametes: a and a
   • Male gametes: A and a
4. Complete the Punnett square:
   • Aa
   • Aa
   • aa
   • aa
5. The affected offspring are aa, so 2 out of 4 offspring have AKU.
6. Probability = 0.5 = 50% = 1 in 2.

✅ Correct answer

Probability that the child will have AKU = 0.5

You could also write 50% or 1/2.

💡 Key knowledge

• AKU is a recessive disorder, so affected individuals are aa.
• The question tells you the male is heterozygous, so he is Aa.
• Gametes carry one allele each.

🧠 Exam technique

• Marks were given for: gametes, correct offspring genotypes, correct offspring phenotypes, and final probability.
• Use the symbols exactly as the question says: A = dominant , a = recessive .
• Always make sure the probability matches the offspring genotype.

❌ Common errors

• Writing the gametes the wrong way round still gets credit if the final offspring are correct.
• Giving 1:2 or 50/50 is not acceptable as the final probability.
• Forgetting that Aa = no AKU and aa = AKU.
• Leaving out units is not an issue here, but the answer must be a probability.

✅ Correct answer

A mutation can cause the wrong sequence of amino acids to be produced. This changes the shape of the enzyme, especially the active site, so the substrate no longer fits.

💡 Key knowledge

• Enzymes are proteins made from amino acids.
• The shape of an enzyme controls its function.
• If the active site changes, the substrate may not bind properly.

🧠 Exam technique

• To get both marks, include:
  1. mutation → incorrect amino acid sequence
  2. shape change → active site changes → enzyme does not work

❌ Common errors

• Simply saying “the enzyme is denatured” is not enough on its own.
• “No enzyme is produced” does not answer the question.
• You must link the mutation to shape and then to function.

✅ Correct answer

1. FSH and LH are given to the female to stimulate egg maturation.
2. The eggs are collected.
3. The eggs are fertilised in the laboratory using sperm.
4. The fertilised egg undergoes mitosis and develops into an embryo.
5. One or two embryos are inserted into the uterus/womb.

💡 Key knowledge

• IVF stands for In Vitro Fertilisation.
• “In vitro” means outside the body, in a laboratory.
• The embryo must develop by mitosis before being placed into the uterus.

🧠 Exam technique

• The marks are for the correct sequence.
• Use short, clear steps.
• Include the idea that fertilisation happens in the lab, not in the body.

❌ Common errors

• Missing out FSH and LH.
• Saying the embryo is inserted before it develops.
• Confusing IVF with natural fertilisation.
• Not stating that the fertilisation happens in the laboratory.

✅ Correct answers

Any two of the following:

• Risk to the embryo
• Risk to the female/mother
• Stressful process
• May lead to termination of the pregnancy
• High cost
• Embryo cannot give consent

💡 Key knowledge

• Screening can involve handling or testing embryos, which may be risky.
• Some embryos may be destroyed if they are found to have a disorder.
• The process can be expensive and stressful.

🧠 Exam technique

• Give two distinct points.
• Develop each point slightly for full clarity, e.g. “high cost” or “may lead to the embryo being destroyed”.
• You do not need to mention religion or ethics here.

❌ Common errors

• Writing only “ethical issues” without explanation.
• Repeating the same idea in different words.
• Talking about religion, which the question says not to do.
• Saying the embryo is harmed “by radiation” — this is not accepted here.

💡 Top-level response features

• Uses evidence directly from the pedigree.
• Knows the difference between heterozygous, homozygous dominant, and homozygous recessive.
• Follows the correct biology sequence in IVF.
• Links mutation to protein shape and enzyme function.
• Gives clear, separate points for evaluation questions.

❌ Biggest mark-losing habits

• Vague explanations with no reference to the diagram.
• Wrong genotype/phenotype links.
• Answering with a list of keywords rather than a linked explanation.
• Forgetting to convert the Punnett square into a final probability.