Punnett squares explained: genetics crosses step by step
TutorAura guide · Updated
A Punnett square is a grid that shows all the possible combinations of alleles in the offspring of two parents. Each box is equally likely, so the grid gives the probability of each genotype and phenotype. Try any cross with the Punnett square calculator.
Key takeaways
- Dominant alleles are capital letters; recessive alleles are lowercase.
- Gametes go along the top and side; offspring fill the boxes.
- Aa × Aa gives a 3 : 1 ratio of phenotypes.
- Ratios are probabilities, not exact predictions.
Key terms
- Gene: a section of DNA that codes for a characteristic.
- Allele: a version of a gene.
- Dominant / recessive: a dominant allele is expressed if present; a recessive one only when there are two copies.
- Homozygous / heterozygous: two identical alleles (AA, aa) or two different ones (Aa).
- Genotype / phenotype: the alleles present / the characteristic that shows.
Drawing the square
Example: two parents are both heterozygous for a characteristic, Aa × Aa.
- Gametes: each parent makes A or a.
- Draw the grid with one parent's gametes across the top and the other's down the side.
- Fill the boxes: AA, Aa, Aa, aa.
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Genotypes: 1 AA : 2 Aa : 1 aa. Phenotypes: 3 dominant : 1 recessive, so a 25% chance of the recessive phenotype.
Inherited disorders
Some disorders are caused by a single gene. Cystic fibrosis is caused by a recessive allele: two carriers (Ff × Ff) have a 1 in 4 (25%) chance of a child with the disorder in each pregnancy. Polydactyly (extra fingers or toes) is caused by a dominant allele, so one affected heterozygous parent (Dd × dd) gives a 50% chance.
Sex determination
In humans, females are XX and males XY. A Punnett square of XX × XY gives XX, XX, XY, XY: a 50% chance of a boy or a girl in each pregnancy. It's the father's sperm, carrying X or Y, that determines the sex.
Family trees
Family trees (pedigrees) show who has a characteristic across generations. If two parents without a condition have a child with it, the condition must be recessive and both parents must be carriers. Work out what you can for certain first, then use Punnett squares for probabilities.
Exam tips
- Define your letters ("F = normal allele, f = cystic fibrosis allele").
- Show the gametes, the grid and the answer.
- Give the answer in the form asked: ratio, fraction, percentage or probability.
Cell division, which makes the gametes, is covered in mitosis vs meiosis.
Practice questions
- What percentage of offspring from Bb × bb will show the recessive phenotype?
- What genotypes come from BB × bb?
- Two parents are carriers of cystic fibrosis (Ff × Ff). What is the chance their child is a carrier?
Answers: 50%; all Bb; 50% (2 of the 4 boxes are Ff).
Frequently asked questions
What is a dominant allele?
An allele that is expressed in the phenotype even when only one copy is present. It's written with a capital letter.
Can two brown-eyed parents have a blue-eyed child?
In simple single-gene models, yes: if both are heterozygous (Bb), each child has a 25% chance of being bb. Real eye colour involves several genes, so it's more complicated.
What is the difference between homozygous and heterozygous?
Homozygous means two identical alleles (BB or bb). Heterozygous means two different alleles (Bb).
What is a carrier?
Someone who is heterozygous for a recessive disorder: they have one recessive allele but don't have the condition. They can pass the allele on to their children.
What does a Punnett square show?
All the possible genotypes of offspring from two parents, and how likely each is.
What ratio does Aa × Aa give?
1 AA : 2 Aa : 1 aa for genotypes and 3 : 1 for phenotypes (dominant : recessive).
Does a 3:1 ratio mean exactly 3 out of 4 children?
No. It's a probability: each child has a 75% chance. Real families can differ, especially when they're small.
Sources
- GCSE Biology (8461) specification: subject content. AQA.
- Cambridge IGCSE Biology 0610 syllabus for 2026, 2027 and 2028. Cambridge International.
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