Multiple Alleles

Mendel worked with genes that came in exactly two alternative forms — a dominant allele and a recessive allele, like tall $T$ and dwarf $t$ in garden pea. But a gene is really just a stretch of DNA, and DNA can mutate more than once at the same site over evolutionary time. When a gene has mutated repeatedly, three or more alternative forms of it can exist across a population, all occupying the same locus on the homologous chromosome pair and all governing the same trait. This phenomenon is called multiple allelism, and the set of alternative forms is called a series of multiple alleles. …