Two-Gene Cross

Follow two genes at once. With both parents hybrid for two traits you get Mendel’s famous 9 : 3 : 3 : 1 — because genes on different chromosomes are passed on independently.

GeneticsDihybrid crossMendel

Inputs

Input examples

Each parent makes four kinds of gametes, one allele of each gene. Both genes are fully dominant and assumed to lie on different chromosomes.

Parent 1

Gene A

Gene B

Parent 2

Gene A

Gene B

The traits

Trait with A (dominant)

Trait with aa

Trait with B (dominant)

Trait with bb

Independent Assortment

Mendel crossed peas that were hybrid for seed shape and seed colour. In the next generation every combination of the two traits appeared, in the ratio 9 : 3 : 3 : 1. That ratio is just 3 : 1 for one trait multiplied by 3 : 1 for the other: the two genes travel independently.

This holds for genes on different chromosomes. Genes close together on the same chromosome tend to be inherited together (linkage), which skews the ratio — the first clue that genes sit on chromosomes at all.

9:3:3:1AaBb × AaBb

1:1:1:1test cross with aabb

16boxes, 9 genotypes

linkagebreaks the ratio

Punnett Square (4 × 4)

AaBb × AaBb

Result

Enter the values above

Key Relations

Independent Assortment

For genes on different chromosomes the two outcomes are independent, so their probabilities multiply.

Example:

Gametes

A parent that is hybrid for both genes makes four kinds of gametes in equal numbers; the square crosses all four with all four of the other parent.

Recessive for Both

A child shows both recessive traits only with two recessive copies of each gene.

Example:

Sources & Verification

References

G. Mendel, “Versuche über Pflanzen-Hybriden”, Verhandlungen des naturforschenden Vereines in Brünn 4, 3–47 (1866).