AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2020: Question 5

💡 Key knowledge

• Females have sex chromosomes XX .
• Males have sex chromosomes XY .
• Egg cells always carry X .
• Sperm cells carry either X or Y .

✅ Correct answers

Male gametes: X and Y

Offspring in the Punnett square:

• XX
• XX
• XY
• XY

🧠 Exam technique

• Read the parent chromosomes first before filling the grid.
• Write one gamete on each row/column label.
• Combine one chromosome from each parent in every box.

❌ Common errors

• Writing XX or YY as sperm.
• Forgetting that the egg can only carry X .
• Putting the chromosomes in the wrong boxes.

✅ Correct answer

There is a 50% chance that the sperm cell fertilising the egg has an X chromosome and a 50% chance it has a Y chromosome.

💡 Key knowledge

The mother always provides an X chromosome. The father’s sperm decides the sex of the baby.

🧠 Exam technique

• Say chance, probability, or 50%.
• Link it to the sperm carrying either X or Y .

❌ Common errors

• Saying the mother chooses the sex.
• Saying it is “random” without explaining why.
• Claiming each child must be different sex.

💡 Key knowledge

• CF is caused by a recessive allele.
• If two unaffected parents have a child with CF, both parents must be carriers.
• Carrier genotype = Tt .
• Only tt children have CF.

✅ Correct answer

Parents: Tt × Tt

Punnett square outcomes:

• TT = unaffected
• Tt = carrier
• Tt = carrier
• tt = CF

Probability of next child having CF: 1 in 4 , or 25% , or 0.25

📐 Calculations / working

1. Because they already have a child with CF, each parent must have one recessive allele.
2. So each parent is Tt .
3. Put the gametes across the top and side: T and t .
4. Fill the square: TT , Tt , Tt , tt .
5. Count the affected children: 1 out of 4.

Answer: 1/4 or 25%

❌ Common errors

• Using TT or Tt for the child with CF.
• Forgetting that both parents must be carriers.
• Giving the ratio but not linking it to the probability.
• Writing “one in three” instead of “one in four”.

✅ Strong reasons you could use

• It does not need IVF, which is stressful and invasive.
• There is a higher chance of successful pregnancy because IVF only has about a 30% success rate.
• The test is free / freely available.
• It can be done once the mother is already pregnant.

💡 Key knowledge

Method 1 = testing the embryo during pregnancy using a sample from the uterus. Method 2 = IVF, testing embryos before implantation.

🧠 Exam technique

• You only need any three valid reasons.
• Make each point different.
• Use comparison words like safer, cheaper, more available, or higher success rate.

❌ Common errors

• Giving reasons that only describe Method 1, without comparing it to Method 2.
• Saying “IVF is bad” without explaining why.
• Repeating the same idea in different words.

✅ Correct answers

• The child will definitely not have CF.
• You do not need to consider termination.
• It can help couples having fertility problems.

💡 Key knowledge

Method 2 uses IVF, so embryos can be tested before implantation and only embryos without CF are put into the womb.

🧠 Exam technique

Pick one clear advantage and explain it simply. The mark scheme accepts any one valid point.

❌ Common errors

• Saying Method 2 is cheaper or easier — this is not supported.
• Forgetting that embryos are screened before implantation.
• Not linking the advantage to CF screening.

💡 Remember

• Sex chromosomes: female XX , male XY .
• Egg = X ; sperm = X or Y .
• CF is recessive; affected genotype is tt .
• Two unaffected parents with an affected child must both be Tt .

🧠 How to gain marks

• Use the correct genetic symbols.
• Show the working in the Punnett square.
• State probability as a fraction, percentage, or ratio.
• For explanation questions, give clear reasons, not just one-word answers.