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IGCSEBiology
Trial Class

Original practice · 5 questions · 16 marks · about 18 minutes

Genetic Diagrams and Punnett Squares Practice Questions

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

These questions practise the technique of drawing genetic diagrams correctly and interpreting the predicted outcomes.

Before you open the answers

  • Attempt every question without notes.
  • Show working for calculations and write complete biological statements.
  • Use the suggested time as a guide, not a strict limit on the first attempt.

These are original practice questions. They do not reproduce an official Cambridge paper or official mark scheme.

  1. Question 1

    Paper 4 Extended 4 marks Difficulty 3/5

    Cystic fibrosis is caused by a recessive allele (f). Two healthy parents each carry one copy of the allele. Use a genetic diagram to determine the probability of their child having cystic fibrosis.

    Show model answer and marking guidance
    1. Both parents are carriers: Ff × Ff.
    2. Punnett square: FF, Ff, Ff, ff.
    3. Probability of cystic fibrosis (ff) = 1 in 4 / 25%.
    4. Probability of being a carrier (Ff) = 2 in 4 / 50%.

  2. Question 2

    Paper 4 Both 3 marks Difficulty 3/5

    A couple has four children, all with brown eyes. Does this prove that both parents are homozygous for brown eyes? Explain your answer.

    Show model answer and marking guidance
    1. No: this does not prove homozygosity.
    2. If both parents are Bb (heterozygous), there is a 3/4 chance each child has brown eyes.
    3. With only four children, it is possible all happen to inherit at least one B allele: predicted ratios are based on probability, not certainty.

  3. Question 3

    Paper 4 Extended 3 marks Difficulty 4/5

    A test cross is used to determine the genotype of an organism showing the dominant phenotype. Explain how this works.

    Show model answer and marking guidance
    1. Cross the unknown organism with one that is homozygous recessive.
    2. If any offspring show the recessive phenotype, the unknown parent must be heterozygous.
    3. If all offspring show the dominant phenotype, the unknown parent is likely homozygous dominant.

  4. Question 4

    Paper 2 Core 2 marks Difficulty 2/5

    State what is meant by the term "dominant allele".

    Show model answer and marking guidance
    1. An allele that is expressed / shows its effect in the phenotype.
    2. Even when only one copy is present (heterozygous condition).

  5. Question 5

    Paper 4 Extended 4 marks Difficulty 5/5

    Sickle cell anaemia is caused by a recessive allele (HbS). A man who is a carrier (HbA HbS) marries a woman who is also a carrier. Using a genetic diagram, calculate the probability that their first child will: (a) have sickle cell anaemia, (b) be a carrier, (c) be unaffected.

    Show model answer and marking guidance
    1. Cross: HbA HbS × HbA HbS.
    2. (a) Sickle cell (HbS HbS) = 1/4 = 25%.
    3. (b) Carrier (HbA HbS) = 2/4 = 50%.
    4. (c) Unaffected (HbA HbA) = 1/4 = 25%.

Mark the set consistently

Award a point only when the idea is clearly present in the written answer. Correct any missed point in a complete sentence, then test the same skill on a different question several days later.

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