Predicting a child’s genotype
A Punnett square, named after British geneticist Reginald Punnett, is a simple grid that shows every way two parents’ alleles can combine. Each parent passes on one of their two copies of a gene, at random. For a single gene with a dominant allele A and a recessive allele a, the square gives the probability of each genotype in a child.
How it works
- Each parent’s genotype is AA, Aa or aa.
- An AA parent always passes A, an aa parent always passes a, and an Aa parent passes either with a 50% chance.
- P(AA) = P(A from parent 1) × P(A from parent 2)
- P(aa) = P(a from parent 1) × P(a from parent 2)
- P(Aa) = everything else
- For several children: P(at least one aa) = 1 − (1 − P(aa))^n
Worked example: two carriers
Both parents are Aa carriers of a recessive trait.
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
- AA: 25%, Aa: 50%, aa: 25%.
- 75% of children show the dominant trait; 25% show the recessive one.
- With 3 children, the chance that at least one is aa is 1 − 0.75³ = 57.8%.
- The chance that none of the three is aa is 42.2%.
All six crosses
| Parents | AA | Aa | aa |
|---|---|---|---|
| AA × AA | 100% | 0% | 0% |
| AA × Aa | 50% | 50% | 0% |
| AA × aa | 0% | 100% | 0% |
| Aa × Aa | 25% | 50% | 25% |
| Aa × aa | 0% | 50% | 50% |
| aa × aa | 0% | 0% | 100% |
Every child is a new draw
Probabilities apply to each child independently. Two carrier parents who already have an affected child still face a 25% chance with the next one; the earlier result does not use up the odds.
Beyond one gene
- Two genes (dihybrid crosses) give a 4 × 4 square and the classic 9:3:3:1 ratio when the genes are independent.
- Incomplete dominance, as in snapdragon flower color, makes Aa look intermediate.
- Co-dominance and multiple alleles, as in the ABO blood groups, need more than two letters.
- Sex-linked traits such as red-green color blindness depend on which parent passes the X chromosome.
- Most human traits, like height or eye color, involve many genes and the environment, so a single Punnett square cannot predict them.
For questions about inherited conditions in a family, a genetic counselor can interpret test results and actual risks.