Pleiotropy

Pleiotropy (or polypheny according to Siemens' terminology), the phenomenon of multiple effects of a gene on the traits of an organism. The initial successes of Mendelism established the erroneous view that each gene is responsible for only one specific trait of an organism. However, further in-depth study quickly led to the accumulation of facts about the influence of certain genes on a number of traits, and in some cases, it is easy to establish morphological or physiological connections between different traits caused by one gene—for example, in numerous cases, genes that determine a particular pigmentation of certain parts of the organism (flowers, fruits, eyes, fur, skin, etc.) cause the same coloration of other or even all parts of it (stems, internal organs, etc.); in other cases, this connection is much more complex and sometimes cannot yet be fully explained. Thus, in oats, the gene causing yellow color of the glumes simultaneously delays the development of awns; the dominant gene causing yellow coloration in mice also causes the development of obesity; the gene for white eyes in the fruit fly Drosophila affects the shape of the internal sexual 40« apparatus. Finally, each gene is characterized by a specific effect on the viability and fertility of the organism: the gene determining white coloration of flowers in the snapdragon (Antirrhinum) simultaneously causes retarded growth and low resistance to cold and parasites; in Drosophila, it has been established that any mutant race differs in a certain average lifespan. The degree of a gene's influence on viability turns out to be extremely varied, up to a lethal effect: the aforementioned gene for yellow coloration in mice leads to the death of the organism in the homozygous state already at the embryonic stage (see Lethal genes). In general, the 'primary', guiding trait characterizing any given gene can only be the most readily detectable one, and therefore it is never possible to firmly limit the effect of a gene. Another aspect of the multiple action of genes is that each trait is the product of the action of many genes: thus, in the fruit fly Drosophila, as many as 15 groups of allelic genes (with a total number of over 50) have been found to determine only eye color. Moreover, numerous, extremely precise studies of the influence of various genes on the enhancement, weakening, and modification of various traits have revealed that, in addition to the many more or less 'specific' genes, an even larger number of them influence traits to some degree as 'modifiers' of the effect of the main genes. Therefore, it is quite justified to consider Pleiotropy from a broader perspective—as the doctrine of the universal interaction of genes. In this understanding, Pleiotropy is an extremely important phenomenon from both methodological and theoretical standpoints. It is precisely the doctrine of Pleiotropy that provides the key to understanding how the 'mosaic' of genes leads to the formation of an organism as a whole. The development of an organism is a complex but integral process, and regardless of what importance is attributed to the role of genes in this process, we must undoubtedly admit their influence at the most diverse stages and in the most varied directions. Therefore, the development of each organ, part, trait of an organism is under the influence of numerous genes acting differently in time and space. The organism as a whole is the result of the interaction of all genes, of the entire genotype under specific conditions of development. Thus, to explain the interaction of genes, no assumption of any additional factors, such as 'super-genes' of the vitalist Driesch, is required. Pleiotropy belongs to the little-studied phenomena, which is partly explained by its complexity. Little is known about the phenomenon of Pleiotropy in humans, although there is every reason to assume its significant role in the complex phenomena of 'polyphenic' inheritance of both normal and pathological traits and diseases (see also Combinations of diseases).