Dihybrid Crosses

A dihybrid cross is an experimental cross between individuals that are homozygous for different versions of two traits. In Mendels experiments involving two traits, he correctly predicted that all F₁ plants would show both of the dominant alleles. For example, all purple flowers and all tall plants. All plants inherited one allele for each trait from each parent. All plants were heterozygous (AaBb).

Mendel wondered if the genes for flower color and plant height would travel together when two F₁ plants were crossed.

The F₂ results showed 9/16 were tall and purple-flowered and 1/16 were dwarf and white-flowered, as were the original parents. However, there were 3/16 each of two new combinations, dwarf purple-flowered and tall white-flowered.

We now know that genes located on nonhomologous chromosomes segregate independently of each other and give the same phenotypic ratio as Mendel observed: 9:3:3:1.

The Mendelian theory of independent assortment states that each gene of a pair tends to assort into gametes independently of other gene pairs located on nonhomologous chromosomes. Mendel concluded that the two "units" for the first trait were to be assorted into gametes independently of the two "units" for the other trait.

Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis. As the number of individual traits increases, the number of genotypes possible in offspring as a result of independent assortment and hybrid crossing increases tremendously.

Mendel presented his results in 1865 but his paper received little notice. He discontinued his experiments in 1871. By 1900, Mendels paper had been rediscovered and his work was finally appreciated.

This animation (Audio - Important) describes dihybrid crosses.

This tutorial (Audio - Important) describes Mendel's theory of independent assortment.

REVIEW: The usual F2 phenotypic ratio of a dihybrid cross is
Answer 9:3:3:1.