dihybrid cross

A dihybrid cross follows two traits at the same time, like watching both a coin and a die on every roll. By tracking two genes together, you can test whether they are inherited independently of one another or whether one travels with the other.

In the textbook version, you cross parents that are heterozygous for both genes and tally the offspring. If the two genes assort independently, the four possible trait combinations appear in the famous 9:3:3:1 phenotype ratio — nine showing both dominant traits, three and three showing one dominant and one recessive, and one showing both recessive traits.

That 9:3:3:1 result is essentially two independent 3:1 ratios multiplied together, and it was Mendel's evidence for independent assortment. The same cross is also a sensitive detector of genetic linkage: when two genes sit close on the same chromosome, the ratio shifts away from 9:3:3:1, with parental combinations over-represented and new combinations rare.