Dominance

dominance

In genetics, the greater influence by one of a pair of genes (alleles) that affect the same inherited trait. If an individual pea plant that has one allele for tallness and one for shortness is the same height as an individual that has two alleles for tallness, the tallness allele is said to be completely dominant. If such an individual is shorter than an individual that has two tallness alleles but still taller than one that has two shortness alleles, the tallness allele is said to be partially or incompletely dominant and the shortness allele is said to be recessive (see recessiveness).

---

Dominance

The expression of a trait in both the homozygous and the heterozygous condition. In experiments with the garden pea, the Austrian botanist Gregor Mendel crossed plants from true-breeding strains containing contrasting sets of characters. For seed shape, round and wrinkled strains were used. When plants with round seeds were crossed to plants with wrinkled seeds (P₁ generation), all offspring had round seeds. When the offspring (F₁ generation) were self-crossed, 5474 of the resulting F₂ offspring were round and 1850 were wrinkled. Thus, the round trait is expressed in both the F₁ and F₂ generations, while the wrinkled trait is not expressed in the F₁ but is reexpressed in the F₂ in about one-fourth of the offspring. In reporting these results in a paper published in 1866, Mendel called the trait which is expressed in the F₁ generation a dominant trait, while the trait which is unexpressed in the F₁ but reappears in the F₂ generation was called a recessive trait. See Mendelism

Traits such as round or wrinkled are visible expressions of genes. This visible expression of a gene is known as the phenotype, while the genetic constitution of an individual is known as its genotype. The alternate forms of a single gene such as round or wrinkled seed shape are known as alleles. In the P₁ round plants, both alleles are identical (since the plant is true-breeding), and the individual is said to be homozygous for this trait. The F₁ round plants are not true-breeding, since they give rise to both round and wrinkled offspring, and are said to be heterozygous. In this case, then, the round allele is dominant to the wrinkled, since it is expressed in both the homozygous and heterozygous condition. Dominance is not an inherent property of a gene or an allele, but instead is a term used to describe the relationship between phenotype and genotype. See Allele, Gene action

The production of phenotypes which are intermediate between those of the parents is an example of partial or incomplete dominance. The phenomenon of incomplete dominance which results in a clear-cut intermediate phenotype is relatively rare. However, even in cases where dominance appears to be complete, there is often evidence for intermediate gene expression.

The separate and distinct expression of both alleles of a gene is an example of codominance. This is a situation unlike that of incomplete dominance or complete dominance. In humans, the MN blood group is characterized by the presence of molecules called glycoproteins on the surface of red blood cells. These molecules or antigens contribute to the immunological identity of an individual. In the MN blood system, persons belong to blood groups M, MN, or N. These phenotypes are produced by two alleles, M and N, each of which controls the synthesis of a variant glycoprotein. In the heterozygote MN, there is separate and complete expression of each allele. This is in contrast to incomplete dominance, where there is an intermediate or blending effect in heterozygotes. Codominance usually results in the production of gene products of both alleles. See Blood groups

Individuals in which the phenotype of the heterozygote is more extreme than in either of the parents are said to exhibit overdominance. The concept of overdominance is important in understanding the genetic structure of populations and is usually related to characteristics associated with fitness, such as size and viability.

The production of superior hybrid offspring by crossing two different strains of an organism is known as heterosis. The hybrid superiority may take the form of increased resistance to disease or greater yield in grain production. The mechanism which results in heterosis has been widely debated but is still unknown. See Breeding (plant), Heterosis

A physiological explanation of dominance was put forward by S. Wright in 1934. He argued that variations in metabolic activity brought about by the heterozygous condition are likely to have little effect on the phenotype because enzymes are linked together in pathways so that the substrate of one enzyme is the product of another. Recessive mutations, when homozygous, may halt the activity of one enzyme and thus bring the entire pathway to a halt, producing a mutant phenotype. Heterozygotes, on the other hand, are likely to have only a reduction in activity of one enzyme which will be averaged out over the entire metabolic pathway, producing little phenotypic effect. Molecular studies of dominance have extended Wright's ideas by exploring the kinetic structure of metabolic pathways and enzyme systems. The results obtained thus far tend to support the thrust of his hypothesis, and have established that the dominant phenotype seen in heterozygotes for a recessive allele can be explained without the need to invoke the existence of modifiers. See Genetics, Molecular biology, Mutation