Allele

Source: Wikipedia, the free encyclopedia.

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

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

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]

Etymology

The word "allele" is a short form of "allelomorph" ("other form", a word coined by British geneticists

phenotypes. It derives from the Greek prefix ἀλληλο-, allelo-, meaning "mutual", "reciprocal", or "each other", which itself is related to the Greek adjective ἄλλος, allos (cognate with Latin
alius), meaning "other".

Alleles that lead to dominant or recessive phenotypes

Main article: Dominance (genetics)

In many cases, genotypic interactions between the two alleles at a locus can be described as

co-dominance and polygenic inheritance.[11]

The term "

carriers" for the mutant allele. 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. Wild type alleles are often denoted by a superscript plus sign (i.e., p+ for an allele p).[12]

Multiple alleles

OCA2 and HERC2. The interaction of multiple genes—and the variation in these genes ("alleles") between individuals—help to determine a person's eye color phenotype. Eye color is influenced by pigmentation of the iris and the frequency-dependence of the light scattering by the turbid medium within the stroma
of the iris.
In the ABO blood group system, a person with Type A blood displays A-antigens and may have a genotype IAIA or IAi. A person with Type B blood displays B-antigens and may have the genotype IBIB or IBi. A person with Type AB blood displays both A- and B-antigens and has the genotype IAIB and a person with Type O blood, displaying neither antigen, has the genotype ii.

A population or

polymorphism) present, or the proportion of heterozygotes in the population. A null allele
is a gene variant that lacks the gene's normal function because it either is not expressed, or the expressed protein is inactive.

For example, at the gene locus for the

ABO blood type carbohydrate antigens in humans,[13] classical genetics recognizes three alleles, IA, IB, and i, which determine compatibility of blood transfusions. Any individual has one of six possible genotypes (IAIA, IAi, IBIB, IBi, IAIB, and ii) which produce one of four possible phenotypes: "Type A" (produced by IAIA homozygous and IAi heterozygous genotypes), "Type B" (produced by IBIB homozygous and IBi heterozygous genotypes), "Type AB" produced by IAIB heterozygous genotype, and "Type O" produced by ii homozygous genotype. (It is now known that each of the A, B, and O alleles is actually a class of multiple alleles with different DNA sequences that produce proteins with identical properties: more than 70 alleles are known at the ABO locus.[14]
Hence an individual with "Type A" blood may be an AO heterozygote, an AA homozygote, or an AA heterozygote with two different "A" alleles.)

Genotype frequencies

Main article: Allele frequency

The frequency of alleles in a diploid population can be used to predict the frequencies of the corresponding genotypes (see Hardy–Weinberg principle). For a simple model, with two alleles;

where p is the frequency of one allele and q is the frequency of the alternative allele, which necessarily sum to unity. Then, p2 is the fraction of the population homozygous for the first allele, 2pq is the fraction of heterozygotes, and q2 is the fraction homozygous for the alternative allele. If the first allele is dominant to the second then the fraction of the population that will show the dominant phenotype is p2 + 2pq, and the fraction with the recessive phenotype is q2.

With three alleles:

and

In the case of multiple alleles at a diploid locus, the number of possible genotypes (G) with a number of alleles (a) is given by the expression:

Allelic dominance in genetic disorders

A number of

hemizygous), they are more frequent in males than in females. Examples include red–green color blindness and fragile X syndrome
.

Other disorders, such as Huntington's disease, occur when an individual inherits only one dominant allele.

Epialleles

While

nucleotide sequence.[15] A specific class of epiallele, the metastable epialleles, has been discovered in mice and in humans which is characterized by stochastic (probabilistic) establishment of epigenetic state that can be mitotically inherited.[16][17]

Idiomorph

The term "idiomorph", from Greek 'morphos' (form) and 'idio' (singular, unique), was introduced in 1990 in place of "allele" to denote sequences at the same locus in different strains that have no sequence similarity and probably do not share a common phylogenetic relationship. It is used mainly in the genetic research of mycology.[18][19]

See also

References and notes

  1. ^ UK: /ˈæliːl/, /əˈliːl/; US: /əˈliːl/; modern formation from Greek ἄλλος állos, "other"
  2. ^ Graur, D (2016). Molecular and Genome Evolution. Sunderland MA (USA): Sinauer Associates, Inc.
  3. PMID 10592272
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  4. PMID 22307690
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  5. PMID 12829588
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  6. S2CID 12076999
    .
  7. ^ Craft, Jude (2013). "Genes and genetics: the language of scientific discovery". Genes and genetics. Oxford English Dictionary. Archived from the original on 29 January 2018. Retrieved 14 January 2016.
  8. ^ Bateson, W. and Saunders, E. R. (1902) "The facts of heredity in the light of Mendel’s discovery." Reports to the Evolution Committee of the Royal Society, I. pp. 125–160
  9. ISBN 978-0-7637-3527-2
    .
  10. PMID 28696921
    .
  11. ^ "Allele". Genome.gov. Archived from the original on 28 June 2021. Retrieved 3 July 2021.
  12. ISBN 978-1-319-21680-1
    .
  13. ^ Victor A. McKusick; Cassandra L. Kniffin; Paul J. Converse; Ada Hamosh (10 November 2009). "ABO Glycosyltransferase; ABO". Online Mendelian Inheritance in Man. National Library of Medicine. Archived from the original on 24 September 2008. Retrieved 24 March 2010.
  14. PMID 12014997
    .
  15. S2CID 8654616
    .
  16. PMID 12127774
    .
  17. S2CID 36938621
    .
  18. PMID 1398049
    .
  19. S2CID 10818930
    .

External links

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