AQA GCSE Combined Science: Trilogy Biology Paper 2 (Higher), November 2021: Question 6

✅ Model Answer

Having two different alleles for a gene (or characteristic).

OR: Having one dominant allele and one recessive allele for a gene.

💡 Key Knowledge

• Alleles are different versions or forms of the same gene.
• Heterozygous: different alleles (e.g. Hh ).
• Homozygous: two identical alleles (e.g. HH or hh ).

❌ Common Errors

• Writing just "different genes" instead of "different alleles" (genes and alleles are NOT the same).
• Writing only an example like Hh without an explanation — the mark scheme explicitly says: ignore examples unqualified.
• Vague statements like "having two different types of DNA".

🧠 Exam Technique

Always state the word allele and mention for a gene / trait. For a 1-mark definition question, be precise and concise.

✅ Model Answer & Punnett Square

Parental Genotypes:

• Father (Person 7): hh
• Mother (Person 8): Hh

Offspring Phenotypes:

• hh = has the disorder
• Hh = does not have the disorder (unaffected)

📐 Step-by-Step Probability Calculation

1. Probability of having the disorder:
   Out of 4 outcomes, 2 are hh .
   Probability = 2 ÷ 4 = 0.5 (or 50% or 1/2)
2. Probability of the child being male:
   Biological sex is determined independently (XX vs XY).
   Probability = 0.5 (or 50% or 1/2)
3. Combined probability (male AND disorder):
   Probability = 0.5 × 0.5 = 0.25
   (Acceptable forms: 0.25, 25%, 1/4, 1 in 4, or 1:3 ratio)

🧠 Exam Technique: Securing all 6 Marks

• Mark 1: Identify Person 7 as hh
• Mark 2: Identify Person 8 as Hh
• Mark 3: Correct offspring genotypes inside the grid ( Hh , Hh , hh , hh )
• Mark 4: Match both genotypes to phenotypes ( hh has disorder, Hh does not)
• Mark 5: Probability of having disorder = 0.5
• Mark 6: Multiplied by 0.5 for male = 0.25

❌ Common Errors

• Forgetting the sex condition: Many students stop at 0.5 (the chance of inheriting the disorder) and forget the question asked for a male child who has the disorder.
• Incorrect ratio format: If expressing 1 in 4 as a ratio, it is 1:3 (1 affected : 3 unaffected). Writing 1:4 as a ratio gets zero marks for probability. Decimals ( 0.25 ) or percentages ( 25% ) are much safer!
• Not labelling which parent is which.

✅ Model Answer (4 Marking Points)

1. The condition was caused by a mutation (a random change in the DNA base sequence / genetic code).
2. The mutation occurred during meiosis (during the formation of gametes / egg or sperm cell).
3. This caused a change in the amino acid sequence of the coded protein.
4. This resulted in a different (or non-functional) protein / enzyme being produced (which caused extra digits to develop).

💡 The Biological Chain of Cause and Effect

• DNA base sequence changes (mutation).
• Occurs during gamete formation (meiosis) so it can be passed to offspring even if parents' body cells lack it.
• Changes mRNA codon sequence, so a different sequence of amino acids is assembled at ribosomes.
• The protein folds differently, producing an altered or novel protein/enzyme that changes phenotype.
• Note: Polydactyly is caused by a dominant allele, so only a single mutated allele is needed for the child to display the disorder.

🧠 Exam Technique: "Explain how..."

When an exam question asks how an inherited condition suddenly appears in a child when neither parent has it in their body cells, follow the genetic hierarchy:

Mutation in gametes → Meiosis → Amino acid order → Altered protein

❌ Common Errors

• Saying the parents were "carriers": Polydactyly is dominant. If parents had the allele, they would already have extra digits! The question explicitly stated neither parent had the allele in their body cells.
• Confusing mitosis and meiosis: The mutation must happen during the production of sex cells (meiosis) or gametes to be passed on from healthy parents.
• Vagueness: Saying "it changed the gene" without naming mutation, or saying "it changed the cell" without mentioning amino acids and protein.