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# Allele

## From Wikipedia, the free encyclopedia

An allele (/əˈll/)[1][2] is a variant form of a given gene.[3] Sometimes, the presence of different alleles of the same gene 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. In this case the chromosomes can be paired: each pair is made up of two chromosomes of the same type, known 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.

### YouTube Encyclopedic

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• ✪ Alleles and Genes
• ✪ What is an allele ? ( Allele examples )
• ✪ Alleles and genes
• ✪ Genes vs Alleles
• ✪ What is an ALLELE ?

#### Transcription

Captions are on! Click CC at bottom right to turn off. Keep up to date by following us on Twitter (@AmoebaSisters) and Facebook! I don’t remember which grade it was where I learned something about my tastebuds that can never be unlearned, but the event and the lesson with genetics has stuck with me forever. For you see, I learned that my tastebuds cannot taste PTC. Let me preface this with explaining that PTC stands for this –we’ll stick with PTC---and it’s a chemical that can be sold on these paper strips. It can be purchased under the name PTC paper, and it is popular in genetic classes because it has this fascinating quality: some people put it on their tongue and immediately say, “Yuck, this is bitter!” And some people, when they place the paper on their tongue…taste absolutely nothing. Well, unless you consider the paper. Does paper have a taste in itself? That’s a debatable question but the point is…some people can taste PTC. Some people cannot taste PTC. And I was really disappointed, because I remember that I was the only one there that could not taste it so here was everyone getting this amazing science experience and I couldn’t taste a thing. Well…. there may have been more than just me that couldn’t taste it in the classroom that day, but they didn’t seem as concerned by the fear of missing out of the PTC paper as I was. I remember someone trying to make me feel better by saying, “Oh, but it tastes bitter! You’re actually lucky.” Then they tried to describe what it tasted like to me. But it’s not the same; I guess I’ll never know for myself what it would have tasted like. Of course, the reason PTC paper is used in genetic classes is because the trait of being able, or not being able, to taste PTC is based on genetics! A reminder from our intro to heredity unit that genes are portions of DNA, and they have the ability to code for a characteristic--- a trait. Like being able to taste, or not to taste, PTC. Now we do want to point out that many traits are actually coded for by interactions of more than one gene. Like eye color, which is quite complex, and determined by interactions of many genes together. In fact, the ability to taste PTC or not, may involve some other gene interactions. There’s even different ranges for how bitter the chemical may taste because there may be more kinds of alleles than we’ll mention---more about that later. But since we do know that the ability to taste PTC or not taste PTC is at least heavily impacted by a specific gene, it does make it powerful for genetic classes. One thing I found so interesting is that my parents can both taste PTC. So why can’t I? Recall that humans have 46 chromosomes. Chromosomes are made up of DNA and protein. It’s a condensed unit of DNA. My whole genetic code is represented by these chromosomes. You inherit 23 chromosomes from your mother and 23 chromosomes from your father. Here's all 46 of them right here. As you can see, there are 23 chromosome pairs. Each pair has one chromosome from one parent and one chromosome from my other parent. If we focus on one of these pairs of chromosomes where the PTC taste sensitivity gene may be found, we can see an area where the PTC taste sensitivity gene could be. Let’s assume this is the locus where the PTC taste sensitivity gene is found---see how it is pointing to a specific area here? That’s because it’s on an area on the chromosomes that refers to a specific gene that codes for a trait. Now, remember how this chromosome is from mom. This one is from dad. Each parent contributes an allele---which is a variant of a gene. An allele is a variety of a gene; a form of a gene. The alleles could be the same form of the gene or different forms of the gene---but regardless, in this case, they’re forms of the gene involved with PTC taste sensitivity. So if PTC taste sensitivity is being used as a one gene trait example---and as we mentioned it may not be that be simple---- then your DNA code has a gene related to PTC taste sensitivity. Together the two alleles you inherit, the forms of that gene, determine the trait of tasting PTC or the trait of not tasting PTC. That gene is involved with coding for taste receptors on your tongue and the receptors you have can make a difference for whether you taste PTC or not. The alleles are typically represented by letters. Since this is all about tasting, let’s use the letter T. But wait---it matters whether I represent it as a capital or lowercase letter! If I use a capital letter to represent an allele, it means it’s a dominant allele. If one---or both---of the alleles you inherited for a trait are dominant, then it will be expressed. More about that later. If I use a lowercase letter to represent an allele, that means it’s a recessive allele. Recessive alleles are typically not expressed unless there is no dominant allele present. Now remember that you have two allele copies, so the combinations you can have here could be TT, Tt, or tt. These are called genotypes. Your genetic makeup. Genotypes can help determine a phenotype, which is a physical characteristic. You’ll notice when writing genotypes, I put the capital letters first if it contains a capital letter. That’s not because the order matters; it’s a formatting formality that capitals are written first. It turns out that being able to taste PTC is a dominant trait. That means the phenotype, which is a PTC taster, is due to a genotype that includes at least one dominant allele. So which genotypes can taste PTC then? Well TT can; both of those alleles are dominant. So can Tt, because remember it only takes the presence of one dominant allele. In fact, the only genotype in this simplified example to not be able to taste PTC would be tt. So obviously that is what I am. I am the tt genotype which results in my non-taster phenotype. But my parents can taste PTC... So what genotypes would they have to be? Well if they were both TT, that wouldn’t be possible. If one was TT and one was Tt, that still wouldn’t be possible. Remember you have to get an allele, a form of a gene, from EACH parent. If my parents do taste PTC and I do not, then my parents have the genotype Tt. And their phenotype is PTC taster. Punnett squares can be used to determine the probabilities of offspring having certain genotypes---which then can be used to determine their phenotypes. But Punnett squares are for another Amoeba Sisters video. Before we end, one more thing to mention. In this example, the dominant trait of being able to taste PTC is more common than the recessive trait of not being able to taste PTC. And one could jump to an assumption that dominant traits are more common, especially since it only takes the presence of one dominant allele to show up in the phenotype. At least, in Mendelian inheritance. But the dominant trait is not always more common in a population, because it's possible that the dominant allele itself is more rare. That can be the case with some forms of polydactyly…that is being born with extra fingers. Some forms of polydactyly can be a dominant trait caused by the presence of at least one dominant allele; however, the dominant allele may not be as common in the population and the condition of having extra fingers is generally rare. Well that’s it for the amoeba sisters and we remind you to stay curious.

## Etymology

The word "allele" is a short form of allelomorph ("other form", a word coined by British geneticists William Bateson and Edith Rebecca Saunders),[4][5] 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 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

In many cases, genotypic interactions between the two alleles at a locus can be described as 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,[6] and the other allele is said to be "recessive". 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 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 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,[7] 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.[8] 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

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;

${\displaystyle p+q=1\,}$
${\displaystyle p^{2}+2pq+q^{2}=1\,}$

where p is the frequency of one allele and q is the frequency of the alternative allele, which necessarily sum to unity. Then, 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:

${\displaystyle p+q+r=1\,}$ and
${\displaystyle p^{2}+q^{2}+r^{2}+2pq+2pr+2qr=1.\,}$

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:

${\displaystyle G={\frac {a(a+1)}{2}}.}$

## 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.[9] 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.[10][11]

## References and notes

1. ^ "allele noun - Definition, pictures, pronunciation and usage notes - Oxford Advanced Learner's Dictionary at OxfordLearnersDictionaries.com". Oxfordlearnersdictionaries.com. Retrieved 29 October 2017.
2. ^ "allele Meaning in the Cambridge English Dictionary". Dictionary.cambridge.org. Retrieved 29 October 2017.
3. ^ Wood, E.J. (1995). "The encyclopedia of molecular biology". Biochemical Education. 23 (2): 1165. doi:10.1016/0307-4412(95)90659-2.
4. ^ Craft, Jude (2013). "Genes and genetics: the language of scientific discovery". Genes and genetics. Oxford English Dictionary. Retrieved 2016-01-14.
5. ^ 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
6. ^ 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.
7. ^ 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.
8. ^ 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.
9. ^ 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.
10. ^ 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.
11. ^ 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.
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