allele

allele (əlēl`): see genetics.

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allele

Any one of two or more alternative forms of a gene that may occur alternatively at a given site on a chromosome. Alleles may occur in pairs, or there may be multiple alleles affecting the expression of a particular trait. If paired alleles are the same, the organism is said to be homozygous for that trait; if they are different, the organism is heterozygous. A dominant allele will override the traits of a recessive allele in a heterozygous pairing (see dominance and recessiveness). In some traits, alleles may be codominant (i.e., neither acts as dominant or recessive). An individual cannot possess more than two alleles for a given trait. All genetic traits are the result of the interactions of alleles.

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allele

any of two or more variants of a gene that have the same relative position on homologous chromosomes and are responsible for alternative characteristics, such as smooth or wrinkled seeds in peas

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allele [ə′lēl]

(genetics)

One of the alternate forms of a gene at a gene locus on a chromosome. Also known as allelomorph.

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Allele

Any of a number of alternative forms of a gene. Allele is a contraction of allelomorph, a term used to designate one of the alternative forms of a unit showing mendelian segregation. New alleles arise from existing ones by mutation. The diversity of alleles produced in this way is the basis for hereditary variation and evolution. The different alleles of a given gene determine the degree to which the specific hereditary characteristic controlled by that gene is manifested. The particular allele which causes that characteristic to be expressed in a normal fashion is often referred to as the wild-type allele. Mutations of the wild-type allele result in mutant alleles, whose functioning in the development of the organism is generally impaired relative to that of the wild-type allele. See Deoxyribonucleic acid (DNA), Gene, Gene action, Genetic code, Mendelism, Mutation

An allele occupies a fixed position or locus in the chromosome. In the body cells of most higher organisms, including humans, there are two chromosomes of each kind and hence two alleles of each kind of gene, except for the sex chromosomes. Such organisms and their somatic cells are said to carry a diploid complement of alleles. A diploid individual is homozygous if the same allele is present twice, or heterozygous if two different alleles are present. Let A and a represent a pair of alleles of a given gene; then A/A and a/a are the genetic constitutions or genotypes of the two possible homozygotes, while A/a is the genotype of the heterozygote. Usually the appearance or phenotype of the A/a individuals resembles that of the A/A type; A is then said to be the dominant allele and a the recessive allele. In the case of the sex chromosomes, one sex (usually the male in most higher animals, with the exception of birds) has only one X chromosome, and the Y lacks almost all of the genes in X. The male thus carries only one dose of X-linked genes and is said to be hemizygous for alleles carried on his X chromosome. As a result, if a male inherits a recessive mutant allele such as color blindness on his X chromosome, he expresses color blindness because he lacks the wild-type allele on his Y chromosome. See Chromosome, Sex-linked inheritance

In a population of diploid individuals, it is possible to have more than two alleles of a given gene. The aggregate of such alleles is called a multiple allelic series. Since genes are linear sequences of hundreds or even thousands of nucleotide base pairs, the potential number of alleles of a given gene which can arise by base substitution alone is enormous.