Changes

An '''allele''' is a variant form of a [[gene]]. Since many organisms have two [[chromosomes|chromosome]], they each have two alleles. Individuals that are homozygous possess two of the same allele, individuals that are heterozygous contain two different alleles.

An '''allele''' is a variant form of a [[gene]]. Since many organisms have two [[chromosomes|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==

==Classification of 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.

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 is present, and so an allele may be recessive in some cases, and dominant (or co-dominant) in another.

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.

====Example====

====Example====

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 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.

===Morphy===

===Morphy===

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.

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.

[[Category:Genetics]]

[[Category:Genetics]]