allele

—allelic /euh lee"lik, euh lel"ik/, adj. —allelism, n.

/euh leel"/, n. Genetics.

any of several forms of a gene, usually arising through mutation, that are responsible for hereditary variation.

[1930-35; < G Allel, appar. as shortening of G equivalents of ALLELOMORPH or allelomorphic gene; allelo- < Gk allelo-, comb. form of allélon of/to one another, reciprocally]

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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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▪ biology

also called  Allelomorph,  

      any one of two or more genes that may occur alternatively at a given site (locus) on a chromosome. Alleles may occur in pairs, or there may be multiple alleles affecting the expression (phenotype) of a particular trait. If the 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. In some traits, however, alleles may be codominant—i.e., neither acts as dominant or recessive. An example is the human ABO blood system; persons with type AB blood have one allele for A and one for B. (Persons with neither are type O.) See also dominance; recessiveness.

      Most traits are determined by more than two alleles. Multiple forms of the allele may exist, though only two will attach to the designated gene site during meiosis. Also, some traits are controlled by two or more gene sites. Both possibilities multiply the number of alleles involved. All genetic traits are the result of the interactions of alleles. Mutation, crossing over, and environmental conditions selectively change the frequency of phenotypes (and thus their alleles) within a population.

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