Punnett square calculator: genotypes and ratios
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Enter each parent's genotype, like Aa, to draw the Punnett square and see the chance of each genotype and phenotype in the offspring. Use a capital letter for the dominant allele and a lowercase letter for the recessive one. It handles one gene (monohybrid crosses, as in GCSE and IGCSE) and two genes (dihybrid crosses).
How to read a Punnett square
- Write one parent's possible gametes (alleles) across the top and the other's down the side.
- Fill each box with one allele from the top and one from the side.
- Each box is an equally likely offspring genotype.
For Aa × Aa, the boxes are AA, Aa, Aa and aa: a genotype ratio of 1 AA : 2 Aa : 1 aa and a phenotype ratio of 3 dominant : 1 recessive.
The key words
- Allele: a version of a gene.
- Dominant: shown in the phenotype if at least one copy is present (capital letter).
- Recessive: only shown when both alleles are recessive (lowercase).
- Homozygous: two identical alleles (AA or aa); heterozygous: two different alleles (Aa).
- Genotype: the alleles; phenotype: the characteristic you see.
Common crosses
| Cross | Genotypes | Phenotypes |
|---|---|---|
| AA × aa | all Aa | all dominant |
| Aa × Aa | 1 AA : 2 Aa : 1 aa | 3 : 1 |
| Aa × aa | 1 Aa : 1 aa | 1 : 1 |
| AaBb × AaBb | 16 combinations | 9 : 3 : 3 : 1 |
A cross with aa (homozygous recessive) is a test cross: it shows whether an organism with the dominant phenotype is AA or Aa.
Probabilities, not predictions
A 3 : 1 ratio means each offspring has a 75% chance of the dominant phenotype. It doesn't mean exactly three out of every four. In humans, cystic fibrosis is caused by a recessive allele, so two carrier parents (Ff × Ff) have a 25% chance of having a child with the condition in each pregnancy.
Exam tips
Show the parents' genotypes, the gametes, the square and the ratio or probability. Say clearly which letters are dominant. Questions often ask for a percentage, a fraction or a ratio, so check which one is wanted. More in Punnett squares explained.
Frequently asked questions
What does a 3:1 ratio mean?
For every four offspring, on average three show the dominant phenotype and one the recessive. Each offspring has a 75% chance of the dominant phenotype.
What is a test cross?
Crossing an organism with the dominant phenotype with a homozygous recessive (aa) one. If any offspring show the recessive phenotype, the first organism must be heterozygous (Aa).
Are dihybrid crosses on the GCSE?
GCSE and IGCSE courses focus on monohybrid crosses (one gene). Dihybrid crosses with a 9:3:3:1 ratio are usually studied at A level, but the calculator shows them too.
Is anything I type sent to a server?
No. The calculation runs in your browser and nothing is stored.
Sources
- GCSE Biology (8461) specification: subject content. AQA.
- Cambridge IGCSE Biology 0610 syllabus for 2026, 2027 and 2028. Cambridge International.
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