An allele is a variant form of a gene. Since many organisms have two chromosome, they each have two alleles. Individuals that are homozygous possess two of the same allele, individuals that are heterozygous contain two different alleles.
Classification of alleles
There are two classification systems to categorize alleles.
The more common, and less scientifically-meaningful, is the recessive/dominant relationship. A recessive allele must be homozygous in order for the phenotype to be observed, whereas the phenotype caused by a dominant allele may mask recessive alleles.
More complex interactions may occur, such as co-dominance, when two dominant alleles may both manifest a phenotype. The weakness with this classification system is that an allele’s behavior depends on which other alleles are present, and so an allele may be recessive in some cases, and dominant (or co-dominant) in another.
A useful illustrative example is the gene that encodes blood type. The “I” gene encodes a protein on the surface of blood cells. There are three common alleles: I[A], I[B], and I[O]. A human will have two of these alleles. A and B confer blood types A and B, respectively. An individual with two A alleles, or an A and an O allele will be blood type A, while an individual with two B alleles or a B and O allele will be blood type B. Hence, the O allele is recessive to both A and B alleles, and A and B are dominant to O. A and B are co-dominant, so that a person with A and B alleles will have blood type AB.
A system proposed by Hermann Muller, and still in use today, is the classification of morphy.
An amorphic allele, also called a null allele, is the absence of genetic information. This type of allele is usually caused by deletion of a gene, or deletion of enough of the gene so as to render it functionless.
A hypomorphic allele has normal function, but less than normal. This type of allele is usually caused by damage to the gene, or more often, damage to the control or regulatory features of the gene. These often have a normal protein product, but less of it.
A hypermorphic allele has increased gene function. Similar to a hypomorph, these alleles usually have alterations in the control or regulatory features of a gene.
A neomorphic allele has a novel or new function. These are often caused by the fusion of two existing genes, creating a hybrid protein product, or the fusion of one gene’s control or regulatory features with the protein encoding region of another gene, causing a gene to be expressed in a new pattern in an organism
An antimorphic allele inhibits or interferes with the function of another allele, causing a reduction in gene function of a normal allele.
This is the scientifically preferred nomenclature, since an allele’s morphy is dependent upon its relationship to a normal, or “wild-type,” allele. In general, amorphic and hypomorphic alleles are classified as “loss-of-function,” and often coincide with “recessive” alleles; hypermorphic, neomorphic, and antimorphic alleles generally are referred to as “gain-of-function,” and often coincide with dominant alleles.