The 9:3:3:1 dihybrid ratio
When two genes assort independently, a cross between two double heterozygotes (AaBb × AaBb) produces four phenotype classes in a 9:3:3:1 ratio across the 16-cell Punnett square: 9 show both dominant traits, 3 show the first dominant only, 3 show the second dominant only, and 1 shows both recessive traits. Each square in the grid is an equally likely outcome, which is why the probability calculator explains the underlying counts.
How to use this calculator
Enter the total number of offspring you expect or observed. The tool multiplies the 9:3:3:1 ratio (out of 16) by your total to give the expected count in each class. For single-gene crosses, use the Punnett square calculator.
- Totals that are not multiples of 16 give fractional expected counts — that is correct, since expectations are averages rather than whole individuals.
- These are phenotype classes, not genotypes. The 9 double-dominant offspring hide several different genotype combinations.
- Use expected counts as the baseline for a chi-square test against what you actually observed.
Worked example
With 16 offspring, each ratio unit equals 1, so the counts are exactly 9 : 3 : 3 : 1. With 160 offspring, multiply by 10 to expect 90 : 30 : 30 : 10. The famous 9:3:3:1 result is a ratio, and the ratio calculator will scale it to any number of offspring.
Frequently asked questions
- What is a dihybrid cross?
- A cross tracking two genes at once, classically AaBb × AaBb, which predicts how two independently assorting traits combine in the offspring.
- Why is the ratio 9:3:3:1?
- The 16-cell Punnett square contains 9 individuals with both dominant traits, 3 and 3 with one dominant trait each, and 1 with both recessive traits.
- Does this assume independent assortment?
- Yes. The 9:3:3:1 ratio holds only when the two genes are on different chromosomes or far enough apart to assort independently.
- Can I use a different number of offspring?
- Yes. Enter any total and the calculator scales the 9:3:3:1 ratio proportionally, so counts may not be whole numbers.
- What happens if the two genes are linked?
- Linked genes sit close together on the same chromosome and tend to be inherited as a pair, so parental combinations appear far more often than 9:3:3:1 predicts. A clear excess of parental phenotypes in your real data is the usual sign of linkage rather than independent assortment.