Principles of Genetics and Breeding

2.1.4 Dihybrid cross

Later on, Mendel tried to find out how different phenotypic traits behave in relation to each other in the inheritance from generation to generation. For this he crossed two varieties of pea plants differing in two pairs of contrasting characters. When a cross is made between two parents differing in two characters, the cross is known as dihybrid cross. When round yellow plants were crossed with wrinkled green plants all the F₁ were Round Yellow. When F₁ were selfed the F₂ offspring produced were in the ratio of 9:3:3:1.

The irregularity of F­₂ offspring ratio was explained by his second law, Law of independent assortment or recombination of genes. It states that "alleles for different traits are distributed to sex cells independently of one another. The allele Y was associated with allele R in parent but it does not always remain associated with it and, also becomes associated with allele r.

Mendel’s genes which underwent independent assortment are located on different chromosomes which recombine freely in meiosis.