An allele (pronounced /ˈæliːl/ (UK), /əˈliːl/ (US); from the Greek αλληλος allelos, meaning each other) is one of a series of different forms of a genetic locus. The word is a short form of allelomorph ('other form'), which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes. Alleles are now understood to be alternative DNA sequences at the same physical locus, which may or may not result in different phenotypic traits. In any particular diploid organism, with two copies of each chromosome, the genotype for each gene comprises the pair of alleles present at that locus, which are the same in homozygotes and different in heterozygotes. 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 (heterozygosity) in the population.
For example, at the gene locus for ABO blood type proteins in humans , classical genetics recognizes three alleles, IA, IB, and IO, that determines compatibility of blood transfusions. Any individual has one of six possible genotypes (AA, AO, BB, BO, AB, and OO) that produce one of four possible phenotypes: "A" (produced by AA homozygous and AO heterozygous genotypes), "B" (produced by BB homozygous and BO heterozygous genotypes), "AB" heterozygotes, and "O" homozygotes. It is now appreciated 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. An individual with "Type A" blood may be a AO heterozygote, an AA homozygote, or an A'A heterozygote with two different 'A' alleles.
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 genotype the phenotype of the heterozygote most resembles. Where the heterozygote is indistinguishable from one of the homozygotes, the allele involved is said to be dominant to the other, which is said to be recessive to the former. The degree and pattern of dominance varies among loci: for a further discussion see Dominance (genetics).
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 dominant, common, and "normal", in contrast to "mutant" alleles regarded as recessive, rare, and frequently deleterious. It was commonly 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.
The frequency of alleles in a 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, than 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:
In the case of multiple alleles at a diploid locus, the number of possible genotypes (G) possible with a number of alleles (a) is given by the expression:
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 hemizygosity, 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 where an individual inherits only one dominant allele.
An allele (pronounced /əˈliːl/ (US), /ˈæliːl/ (UK)) is a viable DNA (deoxyribonucleic acid) coding that occupies a given locus (position) on a chromosome. Usually alleles are sequences that code for a gene, but sometimes the term is used to refer to a non-gene sequence. An individual's genotype for that gene is the set of alleles it happens to possess. In a diploid organism, one that has two copies of each chromosome, two alleles make up the individual's genotype. The word came from Greek αλληλος = "each other".
An example is the gene for blossom colour in many species of flower — a single gene controls the colour of the petals, but there may be several different versions (or alleles) of the gene. One version might result in red petals, while another might result in white petals. The resulting colour of an individual flower will depend on which two alleles it possesses for the gene and how the two interact.
An allele is an alternative form of a gene (in diploids, one member of a pair) that is located at a specific position on a specific chromosome.
Diploid organisms, for example, humans, have paired homologous chromosomes in their somatic cells, and these contain two copies of each gene. An organism in which the two copies of the gene are identical — that is, have the same allele — is called homozygous for that gene. An organism which has two different alleles of the gene is called heterozygous. Phenotypes (the expressed characteristics) associated with a certain allele can sometimes be dominant or recessive, but often they are neither. A dominant phenotype will be expressed when at least one allele of its associated type is present, whereas a recessive phenotype will only be expressed when both alleles are of its associated type.
However, there are exceptions to the way heterozygotes express themselves in the phenotype. One exception is incomplete dominance (sometimes called blending inheritance) when alleles blend their traits in the phenotype. An example of this would be seen if, when crossing Antirrhinums — flowers with incompletely dominant "red" and "white" alleles for petal color — the resulting offspring had pink petals. Another exception is co-dominance, where both alleles are active and both traits are expressed at the same time; for example, both red and white petals in the same bloom or red and white flowers on the same plant. Codominance is also apparent in human blood types. A person with one "A" blood type allele and one "B" blood type allele would have a blood type of "AB".
(Note that with the advent of neutral genetic markers, the term 'allele' is now often used to refer to DNA sequence variants in non-functional, or junk DNA. For example, allele frequency tables are often presented for genetic markers, such as the DYS markers.) Also there are many different types of alleles.
There are two equations for the frequency of two alleles of a given gene (see Hardy-Weinberg principle).
where is the frequency of one allele and is the frequency of the other allele. Under appropriate conditions, subject to numerous limitations regarding the applicability of the Hardy-Weinberg principle, is the population fraction that is homozygous for the allele, is the frequency of heterozygotes and is the population fraction that is homozygous for the allele.
Natural selection can act on and in Equation 1, and obviously affect the frequency of alleles seen in Equation 2.
Equation 2 is a consequence of Equation 1, obtained by squaring both sides and applying the binomial theorem to the left-hand side. Conversely, implies since and are positive numbers.
The following equation (commonly termed the Lee equation) can be used to calculate the number of possible genotypes in a diploid organism for a specific gene with a given number of alleles.
where is the number of different alleles for the gene being dealt with and is the number of possible genotypes. For example, the human ABO blood group gene has three alleles; A (for blood group A), B (for blood group B) and i (for blood group O). As such, (using the equation) the number of possible genotypes a human may have with respect to the ABO gene are 6 (AA, Ai, AB, BB, Bi, ii). The equation does not specify the number of possible phenotypes, however. Such an equation would be quite impossible as the number of possible phenotypes varies amongst different genes and their alleles. For example, in a diploid heterozygote some genotypes may show complete dominance, incomplete dominance etc., depending of the gene involved.
There are 4 different types of alleles. Dominant, recessive, codominant, and incomplete dominant. Depending on the inheritance of two alleles, a person may therefore end up having a dominant, recessive, codominant, or incomplete dominant trait. In a single-gene trait, only two alleles determine the trait. In a polygenic trait, more than two alleles control the trait.
An example of a dominant and a recessive trait is the (dis)possession of a widow's peak. Those who have a widow's peak are dominant and those who do not have one are recessive.
An example of a codominant trait occurs in certain types of calves (cow's young). Some calves are known as "blue roans" for their appearance of both blue and grey hairs.
An example of an incomplete dominant trait occurs in a pink 4-o'clock flower. When a red flower (dominant) and a white flower (recessive) are crossed , those flowers with a heterozygous genotype for color are pink, showing the incomplete dominance of the red allele.
An example of multiple alleles is blood type. There are three alleles for blood type, A, B, and O. Because of this, people can have blood type A, B, AB, or O. AA or AO results in type A, BB or BO in type B, AB results in AB, and OO results in type O.
Genetic disorders are normally caused by the acquisition of two recessive alleles for a single-gene trait. Genetic disorders such as these include Albinism, Cystic Fibrosis, Galactosemia, Phenylketonuria (PKU), and Tay-Sachs Disease. In these cases the two alleles are autosomal (not sex chromosomes). Other disorders are recessive, but because they are located on the X chromosomes (of which men have only one copy), they are much more frequent in men than in women. One example of such a disorder is the Fragile X syndrome.
Some other disorders, such as Huntington's disease, are caused by the presence of a dominant allele.
Natioinal Geographic Society, Alton Biggs, Lucy Daniel, Edward Ortleb, Peter Rillero, Dinah Zike. "Life Science". New York, Ohio, California, Illinois: Glencoe McGraw-Hill. 2002.
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Typical plants and animals have two sets of chromosomes: one set inherited from each parent, and are described as diploid. So they have two alleles at each gene locus. If the two alleles at the same gene locus are identical, the individual is called a homozygote and is said to be homozygous: if instead the two alleles are different, the individual is a heterozygote and is heterozygous.
In the simplest case, the effect of one allele completely ‘masks’ the other in heterozygous combination: that is, the phenotype produced by the two alleles in heterozygous combination is identical with that produced by one of the two homozygous genotypes. The allele that masks the other is said to be dominant to the latter, and the alternative allele is said to be recessive to the former. This phenomenon is called dominance, an idea which originates in the work of Gregor Mendel, the founder of genetics.
The inheritance of alleles, and their dominance, can be represented in a Punnett square.
In this example, parents have the genotype Bb (capital letters show dominant alleles and lower-case letters to show recessive alleles). If B (capital) is found in their genotype, the flower will be red. Therefore, the only time a flower is not red is when the genotype is bb (there are no capital 'B's).
The probability of the flowers having different genotypes are: BB is 25%, Bb is 50%, and bb is 25%. The phenotype of the flower will always be red if a dominant B is in the genotype. Therefore, there is a 25% chance of getting a flower which is not red, and 75% chance of getting a flower which is red.