Punnett Square Calculator
Genotype and trait odds for one gene from two parent genotypes.
What it does
A Punnett square crosses the two alleles of each parent to list the four equally likely child genotypes. With complete dominance, any child holding at least one A shows the dominant trait, while only aa shows the recessive one.
The tool above takes both parent genotypes. It returns the dominant and recessive trait chances with the single most likely genotype, and the note spells out all three genotype shares.
The default cross of Aa with Aa yields one AA, two Aa, and one aa square: 25%, 50%, 25%. Dominant trait carriers total three of four squares (75%) against one recessive square (25%).
These ratios follow the NHGRI genetics glossaries from the National Human Genome Research Institute with the same single gene examples taught in Khan Academy biology units. Real traits often mix many genes, which moves ratios away from these clean quarters.
How to use
- Pick the genotype of the first parent: AA, Aa, or aa.
- Pick the genotype of the second parent the same way.
- Read the dominant trait chance first for the visible outcome.
- Read the recessive chance beside it; the two always add to 100%.
- Use the note for the full AA, Aa, aa split before drawing conclusions.
Formula
- AA
- Two dominant alleles, dominant trait shows.
- Aa
- One of each, dominant trait still shows.
- aa
- Two recessive alleles, recessive trait shows.
Equal segregation assumed: each parent passes either allele with 50% chance. No mutation, no selection, and no linked genes inside this one gene model.
Worked example
Example: Aa x Aa
Both parents heterozygous.
- Parent gametes: each gives A or a with equal chance.
- Four squares: AA, Aa, Aa, aa.
- Genotypes: AA 1 of 4 = 25%, Aa 2 of 4 = 50%, aa 1 of 4 = 25%.
- Dominant trait: 25 + 50 = 75%. Recessive: 25%.
- Tool shows 75%, 25%, Aa (50%).
Reference table
| Cross | AA / Aa / aa | Dominant trait |
|---|---|---|
| AA x AA | 100 / 0 / 0 | 100% |
| AA x Aa | 50 / 50 / 0 | 100% |
| AA x aa | 0 / 100 / 0 | 100% |
| Aa x Aa | 25 / 50 / 25 | 75% |
| Aa x aa | 0 / 50 / 50 | 50% |
| aa x aa | 0 / 0 / 100 | 0% |
Limits
- Single gene with complete dominance only.
- Ignores linkage, mutation, and selection.
- Many real traits are polygenic and skip these ratios.
- Carrier screening needs lab tests, not this page.
Common mistakes
- Reading 75% dominant as 75% AA instead of AA plus Aa.
- Assuming the first child must match the top odds.
- Applying single gene ratios to polygenic traits like height.
- Forgetting that aa x aa can only make aa.
- Mixing genotype shares with trait shares.
Try it yourself
Quick checks. Answers sit tucked under each question.
AA x aa?
All four squares Aa: 100% dominant, top Aa (100%).
Aa x aa?
Aa 50%, aa 50%: dominant 50%, recessive 50%.
aa x aa?
aa 100%: dominant 0%, recessive 100%.
Questions
What is a Punnett square?
A four square grid crossing each parent two alleles to list child genotypes at equal odds.
Aa x Aa ratio?
1 AA : 2 Aa : 1 aa by genotype, 3 dominant : 1 recessive by trait.
Why is Aa the top genotype here?
Two of four squares land Aa, more than AA or aa alone.
Does 75% guarantee three of four children?
No. Each child rolls independently, so real families vary around the odds.
What about two genes?
That needs a 16 square dihybrid grid with 9:3:3:1 ratios, outside this page.
Are human traits this simple?
Rarely. Most traits mix many genes with environment, so treat this as the teaching model.