Recessive

Recessive, a hereditary trait of an organism, the development of which cannot occur in the heterozygous state of the gene determining it. It is believed that the inability to realize a recessive trait is determined by the action of another allelic gene that causes a dominant trait (see Dominance). The concept of a recessive trait, like that of a dominant one, was introduced by G. Mendel (1865) and was earlier considered as one of the essential features of hereditary factors (genes). Therefore, the discovery of phenomena of so-called intermediate inheritance, in which recessive traits manifest in one way or another in heterozygous organisms, was regarded as establishing a special hereditary regularity. At present, it is clear that the basic regularities established by Mendel (see Mendelism) concern the phenomena of transmission and combination of genes in crossing, while the phenomena of realization of gene effects, in which dominance and recessiveness are observed, are subject to special complex regularities. Therefore, it is advisable to speak not of a recessive gene, but of a recessive trait. Thus, first of all, the recessive trait being studied may not be the only effect of the gene determining it (see Pleiotropy). There are known cases when one of the traits determined by a gene is recessive, while others are simultaneously dominant or intermediate. Then, in a number of cases, allelic genes, in realizing traits, do not exhibit phenomena of dominance and recessiveness at all. Thus, when crossing plants with red flowers and plants with white flowers in Mirabilis and Antirrhinum, the offspring produces pink flowers, i.e., of intermediate coloration. The same is observed in cattle when crossing red Shorthorns with white ones: heterozygotes turn out to be 'roan', because red hairs grow alongside white ones.

The mentioned cases of simultaneous determination by a gene of dominant and recessive traits and the existence of genes that do not exhibit phenomena of dominance and recessiveness at all are among the decisive facts against the so-called presence-absence theory, according to which a recessive trait is determined by the complete absence of a gene (see more on Heredity). Finally, even in relation to clearly recessive traits, in a number of cases, a more thorough analysis (morphological, anatomical, microscopic, chemical, etc.) makes it possible to detect the manifestation of recessive characteristics in a heterozygous organism. In general, there is a whole range of transitions from relatively clear recessive traits to intermediate ones, 'semi-dominant' ones, and to the complete absence of recessiveness. Moreover, recessiveness can be determined to a large extent by external and internal conditions of development. Thus, in the fruit fly Drosophila, the normal oval-shaped compound eyes, consisting of a huge number of facets, are a recessive trait in relation to the gene determining irregular-shaped bar eyes with a sharply reduced number of facets; however, the study of the effect of temperature on the development of heterozygous flies for these genes revealed that with changes in temperature, the degree of dominance changes quantitatively. A whole series of internal conditions affects recessiveness and dominance. There are known cases of the influence of the organism's sex on the dominance and recessiveness of traits. Thus, when Suffolk polled sheep are crossed with Dorset horned sheep, it turns out that rams, heterozygous for the gene determining horn development, are always horned, while ewes heterozygous for the same gene are always polled; thus, the same gene, realizing its effect in the male sex, turns out to be 'dominant', while in the female sex it is 'recessive'; it is assumed that a gene determining baldness in humans behaves in a similar way, which would explain the frequency of bald men and the rarity of bald women. The influence of the entire genotype on phenomena of recessiveness and dominance is very significant. It has been established that in relation to certain genes, the quantity of them determines recessiveness and dominance. Thus, in cases of so-called polysomy, when there are more than two homologous chromosomes, complex relationships between 'dominant' and 'recessive' genes are observed. There are known cases when one gene (a), determining a recessive trait, when present in double number (aa), turns out to be 'dominant' in relation to one allelic gene (A), which determined a dominant trait. But apparently not only allelic genes influence the manifestation and non-manifestation of recessive traits. In a whole series of cases, the influence of non-allelic genes has been detected, leading to the non-manifestation of dominant traits and, conversely, to the dominance of recessive ones. On this basis, very recently a theory has been put forward, according to which the recessive or dominant effect of a gene that arose in nature changes over time as a result of selection acting on the heterozygous form. Thus, if a newly arisen gene determines partially dominant traits unfavorable for the organism, selection leads to the survival of organisms with such a genotype that contains genes neutralizing the unfavorable effect of the newly arisen gene. Therefore, in such a genotype, this gene will behave as 'recessive'. Conversely, if a newly arisen gene determines traits favorable for the organism, selection leads to the fixation of a genotype containing genes causing the change of the gene's effect to completely dominant.'