Mendel's Pea Genes.

Mendel conducted his studies on the transmission of genetic elements from one generation to the next using pea varieties commercially available at that time. He presented segregation data for seven character differences in detail. The molecular basis of five of these character differences is known, round versus wrinkled seeds, yellow versus green cotyledons, green versus yellow pods, colored versus uncolored seed coats, and tall versus short stems. Wrinkled peas available in Mendel's time resulted from a transposon insertion in the gene encoding starch-branching enzyme I. Allelic variants in the gene encoding magnesium dechelatase are known to condition pea seeds with green cotyledons, while yellow pods are conditioned by a deletion variant that disrupts chlorophyll synthase gene function. Cultivars with unpigmented seed coats and white flowers are explained by a splicing defect in a gene encoding a basic helix-loop-helix transcription factor. Short cultivars used by Mendel were deficient in bioactive forms of the phytohormone gibberellin because they carried a missense allele of a gene encoding gibberellin 3-oxidase. The allelic diversity of the pea genes Mendel studied and the genetic heterogeneity of corresponding traits are discussed below. The identification of two of Mendel's genes remains to be formally confirmed.