Dominant and recessive relationships
Dominant and recessive relationships are the best known forms of gene interaction between alleles within a single locus. (See also Gene interactions). Diploid organisms have two alleles from each gene. If both alleles at a given locus are the same, then (from the standpoint of the particular gene) this is termed a homozygote individual, a homozygote. If the two alleles differ, then this is a heterozygote individual, a heterozygote. The manifestation of each allele can depend on the second allele in the given locus for the particular individual. It very often happens that a particular allele is recessive, i.e. it is manifested in the phenotype only when present in two copies in the given individual, i.e. in a recessive homozygote. A dominant allele is the opposite of a recessive allele. Its presence is manifested in the same way both in a carrier of two copies of the given allele, i.e. in a dominant homozygote, and in a heterozygote, i.e. in an individual in which it is present in only one copy. The degree to which semi-dominant alleles, i.e. alleles with partial dominance, are manifested in the phenotype of an individual depends on whether they occur in the genotype of the given individual in one or both copies. In co-dominance, the two alleles present are manifested to the same degree to which they would be manifested in the relevant homozygotes. While, in partial dominance, the degree of manifestation of the two alleles in a heterozygote is less than for one or the other homozygote, in super-dominance, the expression of the given trait is greater in a heterozygote than in either of the two homozygotes. Interactions between alleles of a single locus can be divided schematically only if these alleles are manifested in the degree of the phenotype expression of a simple quantitative trait. For traits of qualitative character, it is mostly possible to differentiate only between dominant and recessive alleles; mutual differentiation of alleles with partial dominance, super-dominance and co-dominance is usually rather difficult or even impossible.
The picture is further complicated by the fact that that there are usually more than two alleles of a single gene and also by the fact that dominance is a relative matter, i.e. the matter of the relationship between two specific alleles of a given gene, rather than an absolute property of a particular allele. Allele a1 can act as dominant in relation to allele a2, allele a2 as dominant in relation to allele a3 and simultaneously allele a3 as dominant in relation to allele a1. So that the subject of dominance and recessivity is even more complicated, it is necessary to point out the fact that a particular relationship between two particular alleles can also depend on the context, i.e. the effects of genes present at other loci can also be important. In the presence of a particular allele at locus B, allele a1of locus A can be dominant in relation to allele a2; in the context of a different allele at locus B, allele a1can, on the other hand, act as recessive towards allele a2
A certain amount of direct and indirect evidence demonstrates that the dominance of alleles is actually a more complex phenomenon that is, itself, the subject of biological evolution (Bourguet 2001). For example, it was repeatedly found that, in the natural population, the most common alleles are usually dominant and, on the other hand, minority alleles are frequently recessive. If, on the other hand, we isolate individuals in the laboratory that bear two newly formed mutated alleles, or if we obtain individuals bearing minority alleles in mutually isolated natural populations, then the relationship of partial dominance is mostly found between their alleles. For different explanation of the phenomenon see also Haldane’s sieve.