An allele[1], or allelomorph, is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule.[2]
Alleles can differ at a single position through single nucleotide polymorphisms (SNP),[3] but they can also have insertions and deletions of up to several thousand base pairs.[4]
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 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 this case, the chromosomes can be paired. Each chromosome in the pair contains the same genes in the same order, and place, along the length of the chromosome. For a given gene, if the two chromosomes contain the same allele, they, and the organism, are homozygous with respect to that gene. If the alleles are different, they, and the organism, are heterozygous with respect to that gene.
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, nearly every living human's phenotype for the ABO gene is some combination of just these six alleles.[5][6]
^UK: /ˈæliːl/, /əˈliːl/; US: /əˈliːl/; modern formation from Greek ἄλλος állos, "other"
^Graur, D (2016). Molecular and Genome Evolution. Sunderland MA (USA): Sinauer Associates, Inc.
^Smigielski, Elizabeth M.; Sirotkin, Karl; Ward, Minghong; Sherry, Stephen T. (1 January 2000). "dbSNP: a database of single nucleotide polymorphisms". Nucleic Acids Research. 28 (1): 352–355. doi:10.1093/nar/28.1.352. ISSN 0305-1048. PMC 102496. PMID 10592272.
^Elston, Robert; Satagopan, Jaya; Sun, Shuying (2012). "Genetic Terminology". Statistical Human Genetics. Methods in Molecular Biology. Vol. 850. pp. 1–9. doi:10.1007/978-1-61779-555-8_1. ISBN 978-1-61779-554-1. ISSN 1064-3745. PMC 4450815. PMID 22307690.
^Seltsam A, Hallensleben M, Kollmann A, Blasczyk R (October 2003). "The nature of diversity and diversification at the ABO locus". Blood. 102 (8): 3035–42. doi:10.1182/blood-2003-03-0955. PMID 12829588.
^Ogasawara K, Bannai M, Saitou N, Yabe R, Nakata K, Takenaka M, Fujisawa K, Uchikawa M, Ishikawa Y, Juji T, Tokunaga K (June 1996). "Extensive polymorphism of ABO blood group gene: three major lineages of the alleles for the common ABO phenotypes". Human Genetics. 97 (6): 777–83. doi:10.1007/BF02346189. PMID 8641696. S2CID 12076999.
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