GCSE & IGCSE biology practice

Punnett squares questions and answers

A Punnett square shows every way the parents' alleles can combine. Each parent gives one allele; each box is equally likely. A capital letter is a dominant allele, a small letter is recessive. Give probabilities as a fraction or a decimal (for example 1/4 or 0.25).

8 questions · Foundation tier · Every answer is checked as you type it, with a full worked solution. Updated 27 Oct 2026.

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Where punnett squares comes up

GCSE (AQA)	Biology and Combined Science, Foundation and Higher: complete Punnett square diagrams, and extract and interpret information from genetic crosses and family trees.
Cambridge IGCSE (0610)	Core: use genetic diagrams to predict monohybrid crosses and phenotypic ratios (1 : 1 and 3 : 1), and use Punnett squares. Supplement: test crosses, codominance and sex linkage.

The method

Write each parent's two alleles: one along the top, one down the side.
Fill in the four boxes with one allele from each parent.
Count the boxes: each is a 1/4 (25%) chance.
Recessive phenotype needs two recessive alleles (e.g. bb).

Punnett squares questions

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Warm-up

Question 1Warm-upFoundation

Two parents are both Bb (B = brown eyes, dominant; b = blue eyes). What is the probability that a child has blue eyes (bb)?

Question 2Warm-upFoundation

A cross is Bb × bb. What fraction of the offspring will show the recessive phenotype?

Exam-style

Question 3Exam-styleFoundation

A cross is BB × bb. What fraction of the offspring are heterozygous?

Question 4Exam-styleFoundation

In a Bb × Bb cross, what is the ratio of dominant to recessive phenotypes?

DominantRecessive

Question 5Exam-styleFoundation

Cystic fibrosis is caused by a recessive allele, f. Two parents are both carriers (Ff). What is the probability that their child has cystic fibrosis?

Question 6Exam-styleFoundation

For the same parents (Ff × Ff), what is the probability that a child is a carrier (Ff)?

Challenge

Question 7ChallengeFoundation

A tall pea plant (Tt) is crossed with a short plant (tt). They produce 400 seeds. How many would you expect to grow into short plants?

Question 8ChallengeFoundation

A mother is XX and a father is XY. What is the probability that a child is a boy?

Common mistakes

Thinking a 3 : 1 ratio means exactly 3 of every 4 children will show it.
Forgetting a heterozygous parent (Bb) can pass on the recessive allele.
Mixing up genotype and phenotype.

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Sources

The questions are our own. The “where it comes up” notes are from each board's published specification, checked on 27 October 2026:

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