Trihybrid Crosses

Mendel further extended his research on pea plants to trihybrid crosses, where the organisms vary in three different traits, for example, plant height - designated here by uppercase or lower case T-, seed shape shown here by the allele R, and seed color - delineated by the letter Y. 

A homozygous dominant plant in this cross will be a tall plant with round, yellow seeds and the genotype uppercase TTRRYY. A homozygous recessive plant will be a short plant with wrinkled, green seeds and the genotype lowercase ttrryy.

When these plants are crossed, all of the F1 generation plants are trihybrids, meaning they are heterozygous for all three traits with the genotype shown here.

The F1 generation plants display the dominant phenotype where all of the plants are tall... with round..., yellow seeds.

When there are three pairs of contrasting characteristics, a Punnet square quickly becomes impractical because there are 64 potential genotypes in the F2 generation.

In such cases, the forked line method is often used instead. Here, the F1 heterozygotes with three pairs of traits are arranged in three rows, where each gene occupies one row.

The alleles for plant height are placed in the first ro