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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
dominance(MARXISM) the ‘dominant element’ within a social formation – be this ideology, politics, or the economy – as determined by the particular requirements of the economic base at a point in time (ALTHUSSER, 1966). Althusser wished to draw attention to the internal complexity of social formations, even though these are determined by the economy ‘in the final instance’. He contrasted this view with the ‘Hegelian’ conception of social ‘totality’.
the form of relationship that occurs between the paired (allelic) hereditary elements, or genes, whereby one gene suppresses the function of the other. The former is called the dominant allele and is designated by a capital letter (for example, A); the second is called the recessive allele and is designated by a lower-case letter (for example, a).
G. Mendel introduced the concept of dominance to genetics. A distinction is made between complete dominance and intermediate dominance (semidominance). In complete dominance, only the effect of the dominant allele is manifested, whereas in intermediate dominance the effects of both the dominant and recessive alleles are expressed in varying degrees. Complete dominance, like complete recessiveness, is rare. The manifestation of any character in the phenotype depends on the genotype—that is, on the action of a number of genes. An allele may be dominant, recessive, or expressed in intermediate forms, according to environmental conditions and the genetic makeup of a population (and, consequently, of the individual).
According to the English scientist R. Fisher, dominance is evolving as a system in which gene modifiers are selected for a given initially semidominant allele. If the initial effect of the allele is unfavorable, it becomes latent, or recessive, in the course of selection. If it is positive, the allele becomes dominant. Change in the dominance of an allele when it is carried into another genotype or is influenced by external conditions can be explained by the action of this system (when the action of the gene modifiers is subject to change). The English biologists J. B. S. Haldane and S. Wright assume that it is those alleles that produce an optimal physiological effect that are selected and fixed as dominants (for example, those that synthesize a certain amount of an appropriate enzyme).
Dominance plays an important role in medicine and agriculture. In case of complete dominance an individual may carry harmful alleles in a recessive state, the presence of which becomes apparent only when they are found as homozygous. These phenomena are analyzed by medical geneticists. Livestock breeders analyze sires by the offspring.
IU. S. DEMIN