Allele Definition

Allele – any of the variations of a gene that may occupy the same position (locus) on a chromosome. The gene controlling a particular trait or function always occupies the same locus on the same chromosome. Genes occur as pairs, with one gene coming from each parent. The pairing determines how the gene’s traits are expressed in the individual. For example, the gene for blood type occurs at region 34 on the long arm of chromosome 9, indicated as 9q34. This gene has three alleles, identified as 9q34IA, 9q34IB, and 9q34i (which geneticists sometimes abbreviate as IA, IB, and i, respectively). These alleles can occur in one of six pairings to produce the blood type A, B, O, or AB.

When the two alleles at the same locus are the same the individual is said to be homozygous for that gene; when the alleles are different the individual is heterozygous. In a heterozygous individual generally one allele is dominant and the other recessive. Occasionally each allele in a pairing has equal dominance, a circumstance called codominance. The 9q34i allele (type O) is recessive; the 9q34IA and 9q34IB alleles (type A and type B) are dominant. When the 9q34IA and 9q34IB alleles pair, their expression is codominant. The possible allele pairings for blood type can produce any of these expressions.

Example Allele Pairings and Expression: Blood Type

Example Allele Pairings and Expression: Blood Type
Allele PairingExpressionBlood Type
IaIa (A+A)
Iai (A+O)
Homozygous dominant
Heterozygous dominant
Type A
IbIb (B+B)
Ibi (B+O)
Homozygous dominant
Heterozygous dominant
Type B
ii (O+O)Homozygous recessiveType O
IaIb (A+B)Heterozygous codominantType AB

For further discussion of alleles within the context of the structures and functions of genetics, please see the overview section “Genetics and Molecular Medicine.”


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