Allele

An allele[1] (or allelomorph) is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule.

A notable example of this is Gregor Mendel's discovery that the white and purple flower colors in pea plants were the result of a single gene with two alleles.

Nearly all multicellular organisms have two sets of chromosomes at some point in their biological life cycle; that is, they are diploid.

In population genetics, nearly every living human's phenotype for the ABO gene is some combination of just these six alleles.

In many cases, genotypic interactions between the two alleles at a locus can be described as dominant or recessive, according to which of the two homozygous phenotypes the heterozygote most resembles.

It is now appreciated that most or all gene loci are highly polymorphic, with multiple alleles, whose frequencies vary from population to population, and that a great deal of genetic variation is hidden in the form of alleles that do not produce obvious phenotypic differences.

[12] A population or species of organisms typically includes multiple alleles at each locus among various individuals.

Recessive genetic disorders include albinism, cystic fibrosis, galactosemia, phenylketonuria (PKU), and Tay–Sachs disease.

The term epiallele is used to distinguish these heritable marks from traditional alleles, which are defined by nucleotide sequence.