Dominance (genetics)
For Dominance in Animal behavior see Dominance (animal behavior)
Dominance in genetics describes a type of relationship between the two versions (alleles) of any one gene in which the traits of one allele (the dominant one) are expressed and the traits of the other (the recessive) are repressed. Since most animals are diploid, one allele of each gene comes from the dam (mother) and one allele comes from the sire (father). If either parent passes on a completely dominant gene, that trait will be found in all the offspring.
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For genes on an autosome (any chromosome other than a sex chromosome), the alleles and their associated traits are autosomal dominant or autosomal recessive. Often the dominant allele codes for a functional protein whereas the recessive allele does not.
A classic example of dominance is the inheritance of seed shape (pea shape) in peas. Peas may be round (associated with allele R) or wrinkled (associated with allele r). In this case, three combinations of alleles (genotypes) are possible: RR, Rr, and rr. The RR individuals have round peas and the rr individuals have wrinkled peas. In Rr individuals the R allele masks the presence of the r allele, so these individuals also have round peas. Thus, allele R is dominant to allele r, and allele r is recessive to allele R (this use of upper case for dominant alleles and lower case for recessive alleles is a widely followed convention).
More generally, where a gene exists in two allelic versions (designated A and a), three combinations of alleles (genotypes) are possible: AA, Aa, and aa. If AA and aa individuals (homozygotes) show different forms of some trait (phenotypes), and Aa individuals (heterozygotes) show the same phenotype as AA individuals, then allele A is said to dominate or be dominant to or show dominance to allele a, and a is said to be recessive to A.
Dominance is not inherent to an allele. It is a relationship between alleles; one allele can be dominant over a second allele, recessive to a third allele, and codominant to a fourth. Dominance differs from epistasis, a relationship in which an allele of one gene affects the expression of another allele at a different gene.[3]
