An allele (/əˈliːl/) is a variant form of a given gene. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation. A good example of this trait of color variation is the work Gregor Mendel did with the white and purple flower colors in pea plants; discovering that each color was the result of a “pure line” trait which could be used as a control for future experiments. However, most genetic variations result in little or no observable variation.
The word "allele" is a short form of allelomorph ("other form", a word coined by William Bateson and Edith Rebecca Saunders), which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes. It derives from the Greek prefix ἀλλήλ, allel, meaning "reciprocal" or "each other", which itself is related to the Greek adjective ἄλλος (allos; cognate with Latin "alius"), meaning "other".
Most multicellular organisms have two sets of chromosomes; that is, they are diploid. These chromosomes are referred to as homologous chromosomes. If both alleles at a gene (or locus) on the homologous chromosomes are the same, they and the organism are homozygous with respect to that gene (or locus). If the alleles are different, they and the organism are heterozygous with respect to that gene....LESS