What is the 9:3:3:1 ratio?

The phenotype ratio for a dihybrid AaBb × AaBb cross with complete dominance and independent assortment: 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb.

What is independent assortment?

Mendel's second law: alleles of different genes segregate independently during meiosis. It holds for genes on different chromosomes (or far apart on the same chromosome).

What if the genes are linked?

Linked genes don't assort independently, so the dihybrid ratio is distorted. The calculator assumes independent assortment, so it's an approximation for closely linked genes.

Can I use uppercase and lowercase letters?

Yes. Conventionally, the same letter in both cases (Aa, Bb). Make sure each pair uses the same letter, otherwise the calculator will reject it.

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NSW · HSCBiology

Dihybrid Punnett square calculator

Two-gene cross (e.g. AaBb × AaBb). Generates the 4×4 grid and the phenotype ratio.

Inputs

Parent 1 genotypeTwo genes, four letters. Uppercase = dominant. Example: AABB, AaBb, aabb.Parent 2 genotype

Assumes the two genes are on different chromosomes (independent assortment) and complete dominance.

Result

Punnett square (4×4)

	AB	Ab	aB	ab
AB	AABB	AABb	AaBB	AaBb
Ab	AABb	AAbb	AaBb	Aabb
aB	AaBB	AaBb	aaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

Phenotype ratio

AB: 9/16 (56%)
Ab: 3/16 (19%)
aB: 3/16 (19%)
ab: 1/16 (6%)

For AaBb × AaBb you should see the classic 9:3:3:1 ratio.

How this calculator works

For each parent, the calculator generates the four (or fewer, if homozygous) possible gametes by independent assortment of the two genes. It fills in every combination, then counts genotypes and phenotypes assuming complete dominance.

Common questions

What is the 9:3:3:1 ratio?: The phenotype ratio for a dihybrid AaBb × AaBb cross with complete dominance and independent assortment: 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb.
What is independent assortment?: Mendel's second law: alleles of different genes segregate independently during meiosis. It holds for genes on different chromosomes (or far apart on the same chromosome).
What if the genes are linked?: Linked genes don't assort independently, so the dihybrid ratio is distorted. The calculator assumes independent assortment, so it's an approximation for closely linked genes.
Can I use uppercase and lowercase letters?: Yes. Conventionally, the same letter in both cases (Aa, Bb). Make sure each pair uses the same letter, otherwise the calculator will reject it.