AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), 2023: Question 1

✅ Correct answer

meiosis

💡 Key knowledge

• Egg cells and sperm cells are gametes.
• Gametes are produced by meiosis.
• Meiosis halves the chromosome number.

❌ Common errors

• Writing mitosis instead of meiosis.
• Giving a description instead of the actual term.

✅ Correct answers

You need two reasons. Acceptable answers include:

1. Gametes / eggs / sperm are not genetically identical.
2. There is mixing of genetic information / genes / DNA from both parents.

💡 Key knowledge

• Sexual reproduction involves two parents.
• Each parent passes on genetic information.
• Because gametes are different, offspring inherit different combinations of alleles.

🧠 Exam technique

• For 2 marks, give two separate points.
• Do not just say “the gametes fuse” — that is too vague on its own.
• Use the phrase genetic information from both parents to make the second mark secure.

❌ Common errors

• Saying only “two parents” with no mention of genes or DNA. The mark scheme says this is not enough on its own.
• Saying only “gametes fuse” without explaining mixing of genetic information.
• Repeating the same idea twice in different words.

✅ Correct answer

heterozygous

💡 Key knowledge

• DD = homozygous dominant
• dd = homozygous recessive
• Dd = heterozygous

❌ Common errors

• Writing dominant or recessive. The mark scheme says these are ignored here.
• Misspelling the key term so badly it is unclear.

✅ Correct answers

Parent gametes: from Dd you get D and d ; from dd you get d and d .

Offspring with polydactyly: Dd

Probability child will have polydactyly: 1/2 (also acceptable: 0.5 , 50% , 1 in 2 , 1:1 )

💡 Key knowledge

• D is the dominant allele for polydactyly.
• A dominant allele shows in the phenotype if it is present once or twice.
• So Dd has polydactyly, and dd does not.

📐 Step-by-step method

1. Write the parental genotypes: Dd and dd .
2. Work out the gametes:
   • Dd → D and d
   • dd → d and d
3. Fill in the Punnett square by combining one allele from each parent.
4. Count the offspring with the dominant allele D .
5. 2 out of 4 offspring are Dd , so probability = 2/4 = 1/2 = 50% .

🧠 Exam technique

• Always show the gametes first. This often secures an easy mark.
• Then fill in all four boxes carefully.
• Finally, state which genotype shows the condition and link that to the probability.
• Top answers are clear, methodical, and make the final probability match the Punnett square shown.

❌ Common errors

• Putting D and d as the gametes for dd . This is wrong — dd can only make d gametes.
• Saying the probability is 1:2 . The mark scheme says do not accept 1:2.
• Forgetting that Dd has polydactyly because D is dominant.
• Giving the right probability but no working. You may lose marks for the Punnett square steps.

✅ Correct answers

You need any two of the following:

• It can find out if the embryo has an inherited disorder.
• Parents can prepare for a baby with the disorder.
• Parents can decide whether to continue with the pregnancy.
• Over time, fewer people may have the inherited disorder.

💡 Key knowledge

• Embryo screening looks for alleles linked to inherited disorders.
• It gives information that can help parents and doctors make decisions.
• Preparation could include planning support or early treatment.

🧠 Exam technique

• Make your points clearly separate: write one advantage per line.
• Use the wording inherited disorder to stay close to the mark scheme.
• If you mention preparation, explain it briefly, for example access to early treatment.

❌ Common errors

• Giving disadvantages instead of advantages.
• Being too vague, for example “it helps the parents” without saying how.
• Writing about non-inherited diseases instead of inherited disorders.

✅ 01.1

Meiosis.

✅ 01.2

1. Gametes are not genetically identical.
2. Genetic information from both parents is mixed.

✅ 01.3

Heterozygous.

✅ 01.4

Gametes: D , d and d , d .
Offspring genotypes: Dd , Dd , dd , dd .
The offspring with polydactyly are Dd .
Probability = 1/2 .

✅ 01.5

1. It can find out if the embryo has an inherited disorder.
2. Parents can prepare for a baby with the disorder.

🧠 What strong answers did well

• Used exact terms such as meiosis and heterozygous.
• Explained variation using both different gametes and mixing of genes from both parents.
• Completed the Punnett square logically and made the probability match the genotypes shown.
• Gave two clear, separate advantages for embryo screening.

❌ Where students lost marks

• Confusing meiosis with mitosis.
• Using vague language like “they join together” instead of talking about genes or genetic information.
• Making mistakes with gametes in the Punnett square.
• Not recognising that a dominant allele means Dd shows the disorder.