What Is an Allele

An allele is one of two (or more) alternative forms of any particular gene located on a gene pair.

Gene pairs, found at corresponding positions (loci) on chromosome pairs, represent two alleles. One allele is inherited from the maternal gamete and the other from the paternal gamete.

Many genes Opens in new window have more than one allele, such as those for blood type, with the alleles for types A, B and O all found at the same gene locus.

Additionally, advances in molecular genetics have demonstrated that what were previously considered as simply either dominant or recessive traits, or genes, may actually be produced by a variety of different versions, or alleles of a given gene.

In some circumstances different mutant alleles in the same gene may lead to the same disorder (although the exact clinical picture may vary somewhat), while in other cases different mutant alleles in the same gene may lead to very different disorders. In this latter category, for example, alternate mutations in the fibroblast growth factor receptor genes result in various forms of craniosynostosis, and thus all have a common cause.

Related termsMeaning
PhenotypeThe phenotype is the observable biochemical, physiological or morphological characteristics of a cell or individual resulting from the expression of the cell’s genotype, within the environment in which it is expressed. Allelic variation and expression can affect the phenotype of an organism.
Dominant alleleA dominant allele is the allele of a gene that contributes to the phenotype of a heterozygote.
Recessive alleleThe non-expressing allele that makes no contribution to the phenotype is known as the recessive allele. The phenotype of the recessive allele is only demonstrated in homozygotes who carry both recessive alleles.
Codominant allelesContribute equally to the phenotype. The ABO blood groups are an example of codominant alleles, where both alleles are expressed in an individual.
Partial dominanceIn the case of partial dominance a combination of alleles is expressed simultaneously and the phenotype of the heterozygote is intermediate between that of the two homozygotes. For example, in the case of the snapdragon, a cross between red and white alleles will generate heterozygotes with pink flowers.
HeterogeneityRefers to the phenomenon whereby a single phenotype can be caused by different allelic variants.
HomozygousHomozygous individuals carry two identical alleles of a particular gene.
HeterozygotesAre individuals who have two different alleles of a particular gene. HAPLOTYPE - the term haplotype describes a cluster of alleles that occur together on a DNA segment and/or are inherited together.
Genetic linkageThe tendency for alleles close together to be transmitted together through meiosis and hence be inherited together.
Genetic polymorphismsAre different forms of the same allele in the population. The normal allele is known as the polymorphic or mutant allele. A polymorphism differs from a mutation because it occurs in a population at a frequency greater than a recurrent mutation. By convention, a polymorphic locus is one at which there are at least two alleles, each of which occurs with frequencies greater than 1%. Alleles with frequencies less than 1% are considered as a recurrent mutation. The alleles of the ABO blood group system are examples of genetic polymorphisms. The acronym single nucleoitide polymorphism (SNP) is a common pattern of inherited genetic variation (or common mutation) that involves a single base change in the DNA.
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    Adapted from The Encyclopedia of Genetic Disorders and Birth Defects By James Wynbrandt, Mark D. Ludman