Allele - wikipedia Allele 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 notable example of this trait of color variation is Gregor Mendel 's discovery that the white and purple flower colors in pea plants were the result of `` pure line '' traits which could be used as a control for future experiments. However, most genetic variations result in little or no observable variation. 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. Contents 1 Etymology 2 Alleles that lead to dominant or recessive phenotypes 3 Multiple alleles 4 Genotype frequencies 5 Allelic dominance in genetic disorders 6 Epialleles 7 See also 8 References and notes 9 External links Etymology The word `` allele '' is a short form of allelomorph ( `` other form '', a word coined by British geneticists 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 ''. 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 leading to dominant or recessive, according to which of the two homozygous phenotypes the heterozygote most resembles. Where the heterozygote is indistinguishable from one of the homozygotes, the allele expressed is the one that leads to the dominant phenotype. The degree and pattern of dominance varies among loci. This type of interaction was first formally described by Gregor Mendel. However, many traits defy this simple categorization and the phenotypes are modeled by co-dominance and polygenic inheritance. The term `` wild type '' allele is sometimes used to describe an allele that is thought to contribute to the typical phenotypic character as seen in `` wild '' populations of organisms, such as fruit flies ( Drosophila melanogaster ). Such a `` wild type '' allele was historically regarded as leading to a dominant ( overpowering - always expressed ), common, and normal phenotype, in contrast to `` mutant '' alleles that lead to recessive, rare, and frequently deleterious phenotypes. It was formerly thought that most individuals were homozygous for the `` wild type '' allele at most gene loci, and that any alternative `` mutant '' allele was found in homozygous form in a small minority of `` affected '' individuals, often as genetic diseases, and more frequently in heterozygous form in `` 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. Multiple alleles Eye color is an inherited trait influenced by more than one gene, including 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 I I or I i. A person with Type B blood displays B - antigens and may have the genotype I I or I i. A person with Type AB blood displays both A - and B - antigens and has the genotype I I and a person with Type O blood, displaying neither antigen, has the genotype ii. A population or species of organisms typically includes multiple alleles at each locus among various individuals. Allelic variation at a locus is measurable as the number of alleles ( 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, classical genetics recognizes three alleles, I, I, and i, that determine compatibility of blood transfusions. Any individual has one of six possible genotypes ( I I, I i, I I, I i, I I, and ii ) that produce one of four possible phenotypes : `` Type A '' ( produced by I I homozygous and I i heterozygous genotypes ), `` Type B '' ( produced by I I homozygous and I i heterozygous genotypes ), `` Type AB '' produced by I I 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. 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 ; p + q = 1 ( displaystyle p + q = 1 , ) p 2 + 2 p q + q 2 = 1 ( displaystyle p ^ ( 2 ) + 2pq + q ^ ( 2 ) = 1 , ) where p is the frequency of one allele and q is the frequency of the alternative allele, which necessarily sum to unity. Then, p is the fraction of the population homozygous for the first allele, 2pq is the fraction of heterozygotes, and q 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 p + 2pq, and the fraction with the recessive phenotype is q. With three alleles : p + q + r = 1 ( displaystyle p + q + r = 1 , ) and p 2 + q 2 + r 2 + 2 p q + 2 p r + 2 q r = 1. ( displaystyle p ^ ( 2 ) + q ^ ( 2 ) + r ^ ( 2 ) + 2pq + 2pr + 2qr = 1. , ) 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 : G = a ( a + 1 ) 2. ( displaystyle G = ( \frac ( a ( a + 1 ) ) ( 2 ) ). ) Allelic dominance in genetic disorders A number of genetic disorders are caused when an individual inherits two recessive alleles for a single - gene trait. Recessive genetic disorders include albinism, cystic fibrosis, galactosemia, phenylketonuria ( PKU ), and Tay -- Sachs disease. Other disorders are also due to recessive alleles, but because the gene locus is located on the X chromosome, so that males have only one copy ( that is, they are 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 disease, occur when an individual inherits only one dominant allele. Epialleles While heritable traits are typically studied in terms of genetic alleles, epigenetic marks such as DNA methylation can be inherited at specific genomic regions in certain species, a process termed transgenerational epigenetic inheritance. The term epiallele is used to distinguish these heritable marks from traditional alleles, which are defined by nucleotide sequence. 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. See also Evolutionary biology portal Allozyme Evolution Genealogical DNA test Haplo - sufficiency Meiosis Mendelian error Mendelian inheritance Mitosis Penetrance Polymorphism Punnett square Single - nucleotide polymorphism References and notes Jump up ^ `` allele noun - Definition, pictures, pronunciation and usage notes - Oxford Advanced Learner 's Dictionary at OxfordLearnersDictionaries.com ''. Oxfordlearnersdictionaries.com. Retrieved 29 October 2017. Jump up ^ `` allele Meaning in the Cambridge English Dictionary ''. Dictionary.cambridge.org. Retrieved 29 October 2017. Jump up ^ Wood, E.J. ( 1995 ). `` The encyclopedia of molecular biology ''. Biochemical Education. 23 ( 2 ) : 1165. doi : 10.1016 / 0307 - 4412 ( 95 ) 90659 - 2. Jump up ^ Craft, Jude ( 2013 ). `` Genes and genetics : the language of scientific discovery ''. Genes and genetics. Oxford English Dictionary. Retrieved 2016 - 01 - 14. Jump up ^ 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 Jump up ^ Hartl, Daniel L. ; Elizabeth W. Jones ( 2005 ). Essential genetics : A genomics perspective ( 4th ed. ). Jones & Bartlett Publishers. p. 600. ISBN 978 - 0 - 7637 - 3527 - 2. Jump up ^ 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 16 November 2010. Retrieved 24 March 2010. Jump up ^ Yip SP ( January 2002 ). `` Sequence variation at the human ABO locus ''. Annals of Human Genetics. 66 ( 1 ) : 1 -- 27. doi : 10.1017 / S0003480001008995. PMID 12014997. Jump up ^ Daxinger, Lucia ; Whitelaw, Emma ( 31 January 2012 ). `` Understanding transgenerational epigenetic inheritance via the gametes in mammals ''. Nature Reviews Genetics. 13 ( 3 ) : 155. doi : 10.1038 / nrg3188. PMID 22290458. Jump up ^ Rakyan, Vardhman K ; Blewitt, Marnie E ; Druker, Riki ; Preis, Jost I ; Whitelaw, Emma ( July 2002 ). `` Metastable epialleles in mammals ''. Trends in Genetics. 18 ( 7 ) : 348 -- 351. doi : 10.1016 / S0168 - 9525 ( 02 ) 02709 - 9. Jump up ^ Waterland, RA ; Dolinoy, DC ; Lin, JR ; Smith, CA ; Shi, X ; Tahiliani, KG ( September 2006 ). `` Maternal methyl supplements increase offspring DNA methylation at Axin Fused ''. Genesis. 44 ( 9 ) : 401 -- 6. doi : 10.1002 / dvg. 20230. PMID 16868943. External links Look up allele in Wiktionary, the free dictionary. ALFRED : The ALlele FREquency Database Retrieved from `` https://en.wikipedia.org/w/index.php?title=Allele&oldid=865368184 '' Categories : Classical genetics Genetic genealogy Hidden categories : Use dmy dates from January 2015 Talk Contents About Wikipedia Afrikaans Azərbaycanca Bân - lâm - gú Беларуская Български Bosanski Català Чӑвашла Čeština Dansk Deutsch Eesti Ελληνικά Español Esperanto Euskara فارسی Français Gaeilge Galego 한국어 Հայերեն Hrvatski Bahasa Indonesia Interlingua Íslenska Italiano עברית Basa Jawa Қазақша Kreyòl ayisyen Kurdî Кыргызча Latviešu Lietuvių Magyar Македонски മലയാളം Bahasa Melayu Nederlands 日本 語 Norsk Norsk nynorsk Oʻzbekcha / ўзбекча Plattdüütsch Polski Português Română Русский Scots Sicilianu Simple English Slovenčina Slovenščina کوردی Српски / srpski Srpskohrvatski / српскохрватски Suomi Svenska Tagalog தமிழ் ไทย Türkçe Українська Tiếng Việt Winaray 中文 59 more Edit links This page was last edited on 23 October 2018, at 14 : 19 ( UTC ). 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Question: multiple forms of a gene are referred to as
Answer: allele