Darwin recognized that the mechanisms he was explaining required genetic variability in groups when he developed his ideas of evolution through natural selection. That is, they depended on the distinct characteristics of species within a community and the potential of these distinct characteristics to be handed on to progeny. An allele is a variant of a gene, which is an inherited genetic unit that regulates a certain characteristic of an organism. Mendel, for example, researched a gene that determines the color of pea flowers. This gene has two alleles: a white allele denoted by the letter w, and a purple allele denoted by the letter W. Each pea plant possesses two copies of each gene, which may be identical or distinct alleles. When the alleles are distinct, the dominant allele, W, may obscure the recessive trait, w. The genotype, or collection of alleles, of a plant influences its phenotype, or visible characteristics, in this instance blossom color. The term “allele regularity” refers to the frequency with which an allele occurs in a community. For example, if all alleles in a colony of pea plants were purple alleles, W, then the number of alleles of W would’ve been 100%, or 1.0. If, on the other hand, half the alleles were W and half were w, each allele would have a 50% allele frequency, or 0.5.