Allele

An allele or allelomorph is one of the variant forms of a gene at a particular locus, or location, on a chromosome. Different alleles produce variations in the [Page 60]phenotype of inherited characteristics such as hair color or blood type. Ultimately, virtually all allelic variation arises from differences in DNA base sequence: Either directly in the locus coding for a protein or indirectly through a distant locus, known as the promoter, which is responsible for controlling the level of expression of the gene. Alleles are normally inherited from parents, although acquired mutation can give rise to alleles not present in either parent.

Regardless of how they come to be, allele effects can be classified according to a handful of rules. An allele is said to be “dominant” if the presence of one copy of the allele (out of a possible two) is sufficient to determine phenotype. Conversely, a “recessive” allele must be present in two copies in order to affect phenotype. In reality, there are many alternate states that may violate this simple dichotomy. Codominance exists when two different alleles, present concurrently, both act to determine phenotype. The ABO blood type system is a prime example of codominance where the AB blood type is caused by simultaneous expression of alleles for both A and B antigen. Incomplete dominance exists when heterozygotes (those with two different alleles of a gene) exhibit a phenotype intermediate to the homozygous phenotypes of the two alleles. Finally, incomplete penetrance explains the phenomena where individuals with a particular set of alleles fail to show the phenotype classically associated with that allele.