Dominance

DOMINANCE, a term used to denote the interaction of two allelomorphic genes located in one organism, which is expressed in the fact that the individual exhibits a trait corresponding to only one allele (the dominant one) and does not exhibit the trait corresponding to the other (recessive) allele; e.g., from the marriage of a brown-eyed person with a blue-eyed one, the offspring will be brown-eyed, since the gene determining the trait of brown coloration dominates over its allele—the gene for blue coloration. Both qualitative and quantitative traits can dominate; e.g., in humans, short stature dominates over tall (Davenport), or black color over red. Cases of the dominance of physiological traits are known (e.g., blood clotting dominates over non-clotting). A number of pathological traits also dominate (for example, in humans, polydactyly dominates over the normal number of fingers). Dominance of instincts is observed (e.g., the brooding instinct in chickens dominates over its absence). Dominance can be either complete or intermediate. In the case of complete dominance, a hybrid containing both dominant and recessive genes—in other words, a heterozygote—does not differ in appearance from an individual that received dominant genes from both parents, or a homozygous individual. However, cases are known where, with complete dominance on the outside, it is possible (with more careful investigation) to distinguish the heterozygous form from the homozygous one. With incomplete or intermediate dominance, the first generation of hybrids exhibits the dominant trait in a weakened form; for example, when crossing the white and red races of the plant Mirabilis jalapa (four o'clock flower), a plant with pink flowers is obtained in the first generation. Incomplete dominance can also manifest in the form of so-called variable dominance, consisting in the fact that in F₁, a portion of the individuals exhibits a recessive character. Kellogg observed such cases when crossing silkworms, and Davenport in chickens. Cases of changes in dominance depending on age and external conditions have been noted: e.g., in Drosophila, the gene Beaded (cut wings) is sometimes dominant and sometimes recessive depending on changes in temperature and the alkalinity of the environment. From these examples, it is clear how difficult it is to draw any generalizing conclusion regarding the phenomenon of dominance. E. Baur even expressed the thought that complete dominance does not exist at all, and its assumption is based on the imperfection of our abilities to recognize differences. A number of scientists (Standfuss, de Vries, and especially Correns) believed that the trait that is phylogenetically the youngest always dominates, but this point of view has recently been abandoned. To explain the phenomenon of dominance, a special hypothesis was proposed, bearing the name of the presence-absence hypothesis (or theory). Dominance plays a large role, for example, by hiding a number of harmful recessive genes. Cases of epistasis (see) should be distinguished from dominance, with which it is similar in external manifestation.