An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of
nucleotide
Nucleotides are organic molecules consisting of a nucleoside and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both of which are essential biomolecul ...
s at the
same place on a long
DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chromosomal or genomic location of a gene or any other genetic element is called a locus (plural: loci) and alternative DNA sequences at a locus are called alleles."
The simplest alleles are
single nucleotide polymorphisms
In genetics, a single-nucleotide polymorphism (SNP ; plural SNPs ) is a germline substitution of a single nucleotide at a specific position in the genome. Although certain definitions require the substitution to be present in a sufficiently larg ...
(SNP). but they can also be insertions and deletions of up to several thousand
base pairs.
Popular definitions of 'allele' typically refer only to different alleles within genes. For example, the
ABO blood grouping is controlled by the
ABO gene, which has six common alleles (variants). In
population genetics
Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and pop ...
, nearly every living human's
phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological prop ...
for the ABO gene is some combination of just these six alleles.
Most alleles observed result in little or no change in the function of the gene product it codes for. However, sometimes, different alleles can result in different observable
phenotypic traits, such as different
pigmentation
A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compo ...
. A notable example of this is