GCSE Biology Genetic Inheritance & Punnett Squares Notes
Punnett squares are the standard way to predict genetic outcomes. They work for eye colour, blood group, cystic fibrosis and any single-gene inheritance.
Key concepts
- Monohybrid cross
- A cross involving a single gene with two alleles.
- Homozygous vs heterozygous
- Homozygous = two identical alleles (BB or bb); heterozygous = one of each (Bb).
- Carrier
- A heterozygous individual for a recessive disorder — they do not have it but can pass it on.
- Cystic fibrosis is caused by a recessive allele — both parents must be carriers or sufferers.
- Polydactyly (extra digits) is caused by a dominant allele — one copy is enough.
- Punnett square outcomes are probabilities, not guarantees, in each individual pregnancy.
Worked example
Two carriers of cystic fibrosis (Ff) have a child. Calculate the probability the child has cystic fibrosis.
- 1Ff × Ff Punnett square: FF, Ff, Ff, ff.
- 2Only ff has cystic fibrosis.
- 3Probability = 1 in 4 = 25%.
Answer: 25% (or 1 in 4)
Common mistakes
✗ Assuming a 3:1 ratio means exactly 3 out of every 4 children.
✓ 3:1 is a probability — actual families vary.
✗ Mixing up carrier and sufferer.
✓ Carrier = Ff (no symptoms, can pass on); sufferer = ff (symptoms).
✗ Getting the letters wrong for dominant/recessive.
✓ Use uppercase for dominant, lowercase for recessive — and stick with one letter throughout.
Quick check
Try these, then reveal the answers.
Is cystic fibrosis caused by a dominant or recessive allele?Reveal answer
Recessive.
A cross of Rr × rr. What ratio has the dominant trait?Reveal answer
1 : 1 (50%).
What is a carrier?Reveal answer
A heterozygous individual for a recessive disorder — no symptoms but can pass it on.
Exam tips
- Always label the Punnett square with parents, gametes and offspring genotypes.
- State probabilities as a fraction, ratio and percentage where possible.
- For inherited disorders, name the pattern (dominant/recessive) and give a real example.
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Frequently asked questions
Why can two healthy parents have a child with cystic fibrosis?+
Both parents can be carriers (Ff) — with each pregnancy there is a 1-in-4 chance of an ff child, who has the disorder.
What are the pros and cons of genetic testing?+
Pros: informed reproductive choices, early treatment. Cons: emotional impact, ethical concerns and possible discrimination by insurers or employers.
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