Heredity

HEREDITY, the phenomenon of the transmission to offspring of material factors that determine the development of an organism's traits under specific environmental conditions. The task of studying heredity is to establish the regularities in the origin, properties, transmission, and realization of hereditary factors. At the heart of the oldest, most widespread, and simultaneously least precise definition of heredity lies the understanding of heredity as the phenomenon of the similarity of an organism's traits to the traits of its ancestors. The very fact of similarity long served as a sufficient criterion for concluding that there is a causal dependence between the appearance of traits in offspring and in parental and even earlier forms. Hence the definition of heredity as the phenomenon of transmitting traits to offspring. At present, this definition has lost its meaning, since the criterion of trait manifestation as a necessary condition of the phenomenon of heredity is regarded as naive and erroneous. Historical foundations of the modern doctrine of heredity. Theories of heredity. The first information on heredity arose on the basis of centuries of experience of plant and animal breeders. However, only by the beginning of the 19th century do the first attempts at a special consideration of the phenomenon of heredity belong. These attempts were mainly summaries of facts of heredity scattered in the books of animal breeders, plant breeders, and partly medical books (such are the summaries of I. D. Hofacker, 1828; K. F. Burdach by 1835, and especially P. Lucas, 1847, 1850). The extensive two-volume summary by P. Lucas contains a huge number of facts and a whole series of generalizations or "laws" on types of heredity, the role of both sexes in heredity, sex determination, the heredity of diseases, and much more. In general, by the middle of the 19th century, a vast amount of material on heredity had accumulated, sometimes devoid of elementary criticism. In his work "The Variation of Animals and Plants under Domestication," published in 1868, Charles Darwin collected materials on heredity with extraordinary conscientiousness. However, despite the fact that he was familiar with the writings of Kölreuter, Gärtner, Naudin, and many others, and cites completely clear facts of trait dominance, the phenomenon of segregation (even with numerical data), partly based on his own experiments on Antirrhinum, his generalizations in the field of interpreting specific regularities of heredity do not rise above the level of the epoch. This is partly due to the persistence with which Charles Darwin accumulated facts on the phenomenon of so-called reversion, or atavism (see), as evidence in favor of the origin of cultivated breeds from wild ones. All sorts of facts regarding the combination of traits, the disappearance of recessive traits, and their appearance during segregation were interpreted by him from the standpoint of reversion—"the great principle of heredity." Thus, despite the abundance of material, the phenomena of heredity remained mysterious and the regularities contradictory. It is therefore not surprising that the flourishing of speculative theories of heredity is observed precisely at this time (the beginning of the second half of the 19th century). As is known, Charles Darwin himself, in the same 1868 work, formulated the "temporary hypothesis of pangenesis" (see). Darwin set very broad tasks for the hypothesis—he wanted to explain the phenomena of regeneration, the meaning of fertilization, graft chimeras, individual development, and the entire vast field of heredity, including also the phenomenon of reversion and the inheritance of acquired characters. Darwin's main assumption is that "all units of the body, besides the universally admitted capacity to multiply by means of division, detach from themselves tiny gemmules scattered throughout the entire organism." The gemmules grow, multiply, and accumulate in the germinal elements. All germinal elements (bud, egg, sperm) contain gemmules not only from all the cells of the body, but also resting gemmules of grandfathers, grandmothers, and more distant ancestors. These resting gemmules can manifest themselves after several generations, and then the phenomenon of reversion will be observed. All changes in any cell of the organism during its life are transmitted through correspondingly altered gemmules to the sexual elements. Thus, the theory of pangenesis implied the unconditional recognition of the inheritance of acquired characters (see below). However, in his theory of the representation of body parts, Darwin was not original. It must be emphasized in the most categorical manner that critical remarks regarding Darwin's views on the laws of heredity do not in any way diminish or prejudice Darwin's doctrine of evolution and natural selection, which is an unshakable conquest of modern biology. The immense successes in the study of the regularities of heredity—one of the main factors of natural selection—have brought much clarity to certain debatable questions of evolutionary theory and have further strengthened modern biology on the positions of Darwinism. This is all the more important not to forget, since nowadays in the West, especially in the USA, there are strong tendencies to discredit evolution and Darwinism, and it is sometimes asserted that they allegedly do not correspond to the latest discoveries in biology, in particular genetics. (On heredity as a factor of evolution, see Evolutionary Doctrines.) Such theories were extremely widespread starting from antiquity (Heraclitus, Hippocrates, Democritus). Similar ideas were expressed by Paracelsus. The closest views were expressed by Maupertuis (1748), Hösch (1801), and Erasmus Darwin. In 1871, Galton published the results of testing the pangenesis theory, or rather the proposition that gemmules circulate in the blood. He transfused the blood of black and white rabbits into each other for the supposed transfer of the corresponding gemmules and stated the absence of any effect on the offspring. Darwin objected that the transfer of gemmules by blood is not a mandatory condition, since the theory of pangenesis extends to animals and plants that do not have blood. Experiments with blood transfusions served as an impetus for Galton to create his own theory of heredity (1875), where he attempts to avoid the necessity of the circulation of gemmules. He asserts that the hereditary particles necessary for the development of the individual are already located in the germ products. But in addition, a much larger number of particles remains in a resting state until subsequent generations. In the next generation, again a part of them will go toward the development of the organism, while the greater part will be preserved. This very mass of particles passing from generation to generation was called by Galton the root or rootstock (stirps), whence the entire theory is called the root theory. In this theory, we encounter in embryonic form the theory of the continuity of the germ plasm (see below). Galton was also the first to express ideas close to the theory of amphimixis. Despite the fact that the theory of pangenesis had little success, historically its influence was very great. The concepts of a special hereditary substance embedded in the theory of pangenesis received their development in subsequent theories by Nägeli and especially Weismann. Nägeli's theory (1884) proceeds from the assumption of the structure of plasma from micelles, which he pictures as a special kind of "organic crystals" of a protein nature. Any plasma, according to the character of the arrangement of micelles, can be divided into two parts: a) a mass of plasma with randomly scattered micelles, the so-called "nutritive plasma," or trophoplasm; b) micelles oriented in strict order penetrate the trophoplasm and form a denser plasma—idioplasm. Rows of micelles built into the idioplasm penetrate all cells of the organism. This unified system of idioplasm has in each organism a definite arrangement characteristic only of it; all germinal elements enclose within themselves a part of this idioplasm, the properties of which they reproduce in the offspring. This leads Nägeli, like Darwin, to the conclusion that all changes in the traits of an organism will be transmitted to the idioplasm, and consequently to the idioplasm of the germinal elements; that is, he recognized the inheritance of acquired characters. Nägeli's theory stands somewhat apart in the history of ideas of heredity and did not have a major influence. Weismann's concepts had an immeasurably greater influence on the development of knowledge about heredity. A. Weismann (1834–1914) developed the first foundations of his theory of heredity essentially earlier than Nägeli (starting from 1882); however, his views evolved under the influence of other works and only in 1892 received their completion. Weismann used the first successes of cytology and already in the first drafts of his theory (1885), unlike Nägeli, identified idioplasm with nucleoplasm, i.e., the nucleus, which, incidentally, Strasburger and O. Hertwig had already done in 1884. He created the theory of the continuity of the germ plasm, the first hints of which were expressed by Jäger (1877, 1878) and Nussbaum (1880). He called germ plasm the idioplasm of sex cells, and in this lies the fundamental difference of the latter from body cells, or somatic cells. While somatic cells die simultaneously with the death of the organism, germ cells give rise to a new organism and its own germ cells, that is, they possess potential immortality. Thus, germ cells possess continuous succession, which cannot be said of somatic cells (for details, see Germ Plasm). Furthermore, Weismann created the theory of the structure of idioplasm, i.e., the nucleus.

He views the elementary vital unit as a group of organic molecules and calls them biophores. Biophores are grouped into determinant complexes, which determine the development of particular cells or organs. The set of determinants necessary to create an entire organism was named by him an id. Finally, ids form idants, which Weismann boldly identifies with chromosomes. Thus, chromosomes consist of a large set of ids. To explain the process of organism development and the determination of its properties by the nucleus, Weismann partly utilized the ideas of Roux and de Vries. W. Roux (1883) asserted that mitotic division can be of two kinds: quantitative and qualitative. In the former, the granules of which chromosomes consist are arranged in a linear series and precisely divided in half. In "qualitative" division, the chromatin granules are distributed into unequal groups. According to his view, this conditions the differentiation of cells in the process of individual development. De Vries (1889) created the theory of intracellular pangenesis, in which he also adopted the view that the nucleus is the bearer of heredity. Pangenes are concentrated in the nucleus of sex cells, which, unlike Darwin's gemmules and Weismann's biophores and determinants, are, according to de Vries, representatives not of cells and organs, but of traits and properties of the organism. In the process of organism development, pangenes gradually pass from the nucleus into the plasma and determine the properties and traits of various parts of the organism. It was these ideas about unequal divisions and the partial exit of hereditary primordia into the plasma that Weismann used to explain the determination of the properties of the soma, i.e., the body of organisms. Thus, according to Weismann, the nucleus of somatic cells does not contain a complete set of hereditary units; only the germ plasm possesses this, i.e., the nuclei of sex cells. Despite the fact that all the described theories now possess predominantly historical interest, they determined to a significant degree the further development of the problems of heredity and led to a series of experimental works. Weismann's ideas exerted a particularly fruitful and prolonged influence. However, the fate and theoretical value of Weismann's individual views are very different. In Weismann's views on heredity, the following two main propositions must be distinguished: 1. The doctrine of somatic and germ cells and the resulting assumption of the continuity of the hereditary substance of germ cells, realized through the so-called germ path, i.e., the isolation in the early stages of development of the elements of future sex cells and the preservation of their specificity throughout individual development. 2. The doctrine of the structure of germ plasm (biophores, determinants, ids, idants) and the theory of ontogenetic development based on hereditarily unequal divisions. The first proposition led Weismann to deny the inheritance of acquired characters, which undoubtedly played a huge historical role in the development of the problems of heredity and evolution, since this exposed the untenability of the purely epigenetic and Lamarckian views prevailing at the time. On the other hand, the extreme point of view adopted by Weismann on the question of the change of germ plasm—the complete denial of external environment influence and the theory of amphimixis as the sole cause of changes—led him to reformist and autogenetic views. [However, later (1892) he abandoned amphimixis as the sole cause of changes and recognized that "amphimixis cannot create new changes," and the main cause is "in the direct action of external influences."] The theory of the germ path, after numerous studies, was basically confirmed, although not in the schematic form that Weismann supposed. Apparently, apart from the extreme difficulties of studying the germ path in various forms, there are concrete features and modifications of the determination and isolation of sex primordia. Weismann also admitted certain departures from the scheme, for example, to explain regeneration. Already in the theory of the germ path, Weismann comes to the admission of hereditarily unequal divisions, but the latter proposition received its ultimate completion later (1892), when he came to the conclusion that "ontogenesis is conditioned by the sequential process of disintegration of ids of the germ plasm, which split into smaller and smaller groups of determinants during the development of each individual... In the end, if we leave aside possible complications, in each cell there remains only one kind of determinant, namely the one that regulates the function of this cell or this group of cells. In this cell they disintegrate into their constituent biophores and give it an inherited specific character." This group of Weismann's views provoked sharp criticism and experimental refutations from representatives of experimental embryology, which was experiencing its heyday at that very time. These views did not receive further development. "The application of this theory to embryonic development lies outside the modern theory of heredity, which either completely ignores the process of development or puts forward a point of view completely opposite to Weismann's, namely, that in every cell of the body a complete complex of hereditary primordia is present (excluding cases of elimination or chromosome distribution disturbances)" (Morgan). Finally, Weismann's views on hereditary units as representatives of parts, organs, and cells are completely obsolete, since the modern doctrine of genes views them not as preformed "representatives" and "primordia" of particular parts of the organism, but as modifiers in one respect or another of the process of individual development, acting at the most diverse stages of it. The traits of the adult organism, on the other hand, represent the result of a complex interaction of the most diverse genes, most likely of the genotype under concrete conditions of the development environment (see Pleiotropy). Completely obsolete is also the complex multi-tiered hierarchy of the structure of germ plasm (biophores, determinants, ids, idants). Therefore, it is understandable that the evolution of Weismann's own views has little in common with those trends of modern genetics that are often designated by the term "Weismannism." As usually happens in the historical development of science, the trends of Weismannism were expressed in the adoption of those elements of Weismann's views that for one reason or another seemed to agree with the concrete course of development of empirical research. Such elements were the division of the organism into "soma" and sex cells and the autogenetic trends in Weismann's early views, and these concepts were increasingly schematized and absolutized, all the more since they seemed "legitimate" because they "agreed" with all the discoveries of genetics. The fruitful and methodologically completely correct struggle against Lamarckism and its particular elements (somatic induction, adequate heredity, etc., see below) led many genetics researchers to a complete denial of the organic connection and interaction between the "soma" and sex cells. The latest and most extreme expression of this trend is the views of the group of our Marxist biologists, affiliated with the philosophical revisionist current of Menshevising idealism. Menshevising idealist biologists, inclined to an extreme exaggeration of the methodological role of genetics in biology, not only uncritically assimilated the essentially empiricist and non-dialectic denial of the connection and interaction between the "soma" and sex cells, but gave these views the most schematic and metaphysical expression, viewing the "soma" only as a "case" for sex cells. In these views, they turn out to be uncritical followers of the most non-dialectic, metaphysical representatives of empirical biology. An even more dangerous methodological error of Weismannism, and following it of Menshevising idealist biologists, was the complete denial of external factors of hereditary variability. Again, the empirical results of many years of extremely fruitful struggle of genetics with Lamarckists more and more reinforced many geneticists in autogenetic concepts, which in turn grew into incorrect concepts in matters of evolution (for criticism, see below—Morganism). In general, it should be noted that both these and a number of other fundamental questions and theoretical concepts of modern genetics require critical consideration and methodological clarification and revision from the standpoint of materialist dialectics. (On Mendel and his predecessors, see Genetics, Mendelism.) Inheritance of modifications. Weismann also established the division of traits and properties of the organism into innate and acquired. Acquired traits are those that arise under the influence of the conditions of life and development of the organism. However, the concept of "acquired trait" cannot be considered precise. Every trait develops under certain conditions of environment and development. Therefore, even a knowingly hereditary trait exhibits variation. This variation can be not only quantitative (size, height, weight, etc.), but also qualitative (shape, color, etc.). Any changes caused by the conditions of development and environment are called modifications (see).

Therefore, the problem of the inheritance of acquired characters is an artificially isolated part of the general problem of the inheritance of modifications. Entirely associated with this problem historically is a trend in the study of heredity that is conventionally called the biometric trend. It originates from the works of Quetelet (1835, 1846, 1870), the founder of modern variational statistics. Biometrics was created by Galton (1889), Weldon, and Pearson. Galton investigated the question of the inheritance of variational deviations (see Galton's laws). He studied the inheritance of sweet pea seed size, human height, and other characters. Galton sought to study these characters on the largest possible material in order to construct a variation series with a mean value for each of them. It was extremely interesting to ascertain whether extreme deviations from the mean value are inherited, that is, whether the entire variation series shifts in the progeny of such an extreme specimen in its direction. This question was of great importance, since two problems were resolved simultaneously with its solution. Firstly, this proved the possibility of selecting small variations and their accumulation, which was necessary for Darwin's theory. Secondly, it became possible to express the laws of heredity in precise mathematical formulas. For this purpose, Galton gathered information on the height of 204 pairs of parents and their children with a total number of 928 individuals. Comparing the height of parents and their children, he could mathematically calculate the degree of their coincidence and express it numerically. By this path he established the "law of filial regression," which consists in the fact that the parental character is not inherited fully, but only as ²/₃ of the deviation from the mean value. This incompleteness of inheritance is explained, according to Galton, by the influence of ancestral characters. Therefore, he establishes another "law"—"the law of ancestral inheritance"—which is also of a statistical nature (see Galton's laws). In a similar manner, Galton and his followers studied a multitude of other characters. It is obvious that Galton's laws represent simple descriptions of statistical regularities. However, they created the impression of the heritability of modifications, that is, deviations from the mean value, which confirmed the possibility of the selection of variations. The root of Galton's error was discovered only in 1903 by the brilliant work of the Danish botanist W. Johannsen. Galton's main error lay in the fact that his laws were established on a population, that is, a mixture of randomly taken families with the most diverse hereditary predispositions. Johannsen set himself the goal of verifying Galton's work on material that was as pure and uniform in a hereditary sense as possible. Such a material in plants is the generation obtained by the self-pollination of a single plant. Johannsen proposed calling such a generation a pure line. Bean seeds served as his object. He bred a number of pure lines of beans, that is, the seeds of each such line were collected from a single plant that propagated by self-pollination. He studied, among other things, the weight of the seeds. It turned out that the individual weight of the seed from which one or another pure line arose does not determine the average weight of the latter at all. A pure line with a much greater average weight can originate from a single seed with a small weight; at the same time, another seed of equally small weight can give a pure line with a completely different average weight, for example, an even smaller one. This is explained by the fact that maternal seeds of the same or close weight can originate from various pure lines, in the variation series of which they occupied completely dissimilar positions. And, conversely, very dissimilar seeds can originate from close lines or even from the very same line. These will be the so-called plus- and minus-variants, that is, those most deviating from the average weight of the pure line. Two such seeds, despite their strong difference, will give similar progenies which will repeat the variation series of the pure line from which they both originated. Thus, the inheritance of individual deviations observed by Galton is not observed in pure lines. For this same reason, the selection of pure lines yields no effect. No matter how many generations we select seeds in the direction of increasing or decreasing weight, the variation series of the progeny will be the same with the same mean value. The result obtained by Galton is understandable. If one mixes the seeds of all pure lines and studies the variation of the mixture as a whole, that is, in a population, and conducts selection in it, the same results are obtained as in Galton. Johannsen carried this out and obtained results that were very similar even numerically. The outcome of Johannsen's research was the proposition that modifications are not inherited and that selection in pure lines yields no effect. Johannsen's data have been verified on the most diverse objects and are currently completely indisputable (see Pure lines). The most significant works after Johannsen are those of H. Jennings (1908–16) on protozoa. Instead of pure lines, he had to work with clones. A clone—a term proposed by Shull (1912)—means progeny obtained by asexual reproduction or parthenogenesis from a single individual. The results of Jennings's and others' research on clones fully confirmed Johannsen's assertions (see Pure lines). In higher animals, where progeny cannot be obtained from a single individual, a concept closer to pure lines is the so-called blood line—a term proposed by Pearl (1911)—progeny from inbreeding, that is, sibling mating. It is completely clear that the impossibility of obtaining pure lines in higher animals indicates only methodological difficulties of study and in no way testifies to the inapplicability of Johannsen's doctrine to higher animals and man. Johannsen's doctrine is universal. Inheritance of acquired characters. Although the doctrine of pure lines entirely resolves the whole problem of the heredity of modifications, one still has to consider the question of acquired characters separately. This is explained predominantly by historical reasons—the question of the inheritance of acquired characters arose long before clear methodological ideas about the problem were formed. Moreover, proponents of the heritability of acquired characters considered it necessary for obtaining an effect to apply more drastic changes in external conditions, achievable predominantly by experimental means. As was mentioned, the concept of an "acquired character" is not precise. Such a character can arise first of all thanks to a hereditary change in the germ cell of the ancestor (see below about mutations). In this case, it is hereditary upon its very appearance, and the cause of its heritability relates to the problem of the change in the hereditary substance, that is, to another problem which Siemens proposed to call the problem of idiokinesis. Another category of phenomena relates to cases in which the action of conditions on the organism did not cause any new character in the parent, yet produced a change in the hereditary substance which will manifest itself in the progeny. This type of origin of an "acquired" or rather new character differs from the first only in that here we know the cause of idiokinesis—external influences; in the first case, either the cause is unknown or it took place prior to the experiment (in the ancestor). The third type is that in which the action of conditions on the organism causes a new character in the parent, but at the same time also produces a change in the hereditary substance. This type bears the name of "parallel induction," that is, the action simultaneously on body cells and on germ cells. Finally, the fourth type is the so-called somatic induction, in which the action occurs only on somatic cells and causes an acquired character; if the latter is transmitted to the progeny, one can conclude that the change in the hereditary substance occurred through the influence of the somatic change (acquired character) on the germ cells. It is not difficult to see that the division into four types is conventional and relates to phenomena that can be observed under concrete experimental conditions. If, however, one makes an objective classification, we will establish merely two types. 1. Change in the hereditary substance independently of the acquired characters of the organism. 2. Change in the hereditary substance under the influence of the acquired characters of the organism. The first type combines the 1st, 2nd, and 3rd of those mentioned above and relates to the problem of the change in the hereditary substance, that is, the problem of idiokinesis. The second type represents cases of somatic induction and relates to the problem of the inheritance of acquired characters. Strictly speaking, the problem of idiokinesis entirely engulfs the problem of the inheritance of acquired characters. However, we repeat that historically the development of ideas proceeded differently. Thus, in order to prove the heritability of acquired characters, it is necessary to prove the existence of somatic induction.

The first condition for the possibility of the latter is the proof of the existence of a direct connection between somatic and germ cells, since induction is conceivable only as transmission through some material conductors. The question of the presence of a material connection between somatic and germ cells is decided in the affirmative by many experiments. Especially illustrative are the facts relating to so-called false heredity. First of all, mention should be made of the phenomenon of the transmission of infectious diseases to offspring. It has long been spoken of the "heredity" of tuberculosis, syphilis, and other diseases. In reality, in these cases placental infection (bacilli, toxins) or infection of the germ elements takes place. Ehrlich (1892) observed the transmission of immunity in mice to their offspring. In this case, we are dealing with the introduction of antibodies into the eggs. The transfer of trypanosomes into the egg was observed by Schaudinn (1904) in the mosquito. All such cases are generally called phenomena of blastophthoria (Forel, 1907). By this same type occurs the transmission not only of poisons and disease-causing agents, but also of any elements capable of penetrating the egg, sperm, or embryo. Thus, Sitowski (1905) fed caterpillar food colored with a red dye, as a result of which the hatched butterflies and their eggs were colored red. At the same time, let us touch upon another group of facts of false heredity based on a gross delusion. These include telegony. Telegony is the alleged influence of the traits of a male that previously mated with a female on the offspring obtained from subsequent fertilization by another male. Although this view has long been abandoned, it is sometimes encountered among physicians and livestock breeders. Thus, Orth cites a case where a man suffering from hypospadias had children with hypospadias by his wife; when she subsequently entered into a second marriage with a supposedly healthy man, she also bore children from him who suffered from hypospadias. How critically one must relate to such "facts" is shown by the well-known example of telegony in the horse. An Arabian mare gave birth to a hybrid foal from crossing with a quagga male. In the offspring subsequently obtained from an Arabian stallion, striping was discovered similar to that of the quagga. It turned out that horses often exhibit hereditary predispositions to such striping, which obviously existed in the Arabian mare. Even more ignorant and historically overcome is the notion of the influence of the pregnant woman's psychological experiences on the offspring ("maternal impressions"). Thus, the same Orth describes a case where a woman before conception saw a conserved fetus with a jaw defect. The child born to her allegedly had the same defect. The well-known sexologist Rohleder discusses this case completely seriously! Attempts to prove somatic induction. A large number of works relate to transplantations of gonads from an individual of one breed into an individual of another breed. Guthrie (1908) transplanted the ovary of a white hen to a black hen (having previously castrated it) and vice versa. The black hen with the implanted ovary of the white one produced not only white chicks from a white rooster, but also mottled ones; the white hen with the ovary from the black one, from crossing with a black rooster, produced all mottled chicks. The appearance of mottled chicks was interpreted as the influence of the body traits of the hen on the implanted ovary. Numerous verification works found that castration in the hen is difficult to accomplish completely and that in Guthrie's experiment the regeneration of the hen's own ovary took place. In addition, Castle and Phillips (1911) performed analogous experiments on guinea pigs and obtained a negative result. The flawless experiments of Klatt (1919) on the gypsy moth also yielded no result. At the present time, no one disputes that experiments on gonad transplantation speak against somatic induction. Similar experiments relate to the field of so-called chimeras and grafts. Chimeras are organisms formed from tissues of two individuals fused to one degree or another. It is especially easy to obtain such chimeras in plants. It turns out that despite the close coexistence of the tissues of two species and sometimes the formation of peculiar "hybrids" of an intermediate type, sexual reproduction yields plants that always unequivocally belong to one of the species. Thus, here too the germ cells were not subjected to the influence of somatic cells of another species. Equally unconvincing were experiments on the inheritance of mechanical injuries. Already Weismann (1889) investigated this issue by cutting off the tails of mice for 22 generations, with a negative result. Even earlier, Brown-Séquard, by damaging the central and peripheral nervous system in guinea pigs, caused phenomena close to epilepsy and a number of other phenomena. In the offspring, a number of degenerative phenomena, atrophy, paralysis, organ defects, etc., were discovered. However, careful experiments by Maciesza and Wrzosek (1911) showed that epilepsy is extremely widespread among guinea pigs in general, apart from experimental exposure. To the very recent times belongs the work of Kammerer (1923), who cut the siphons of ascidians. The regenerating siphons became significantly longer. He observed the same in the offspring. Munro Fox (1924) verified Kammerer's work and found that the elongation of the siphons depends on the nature of the diet of the ascidians. In general, the inheritance of mechanical injuries is nowadays defended by almost no one, since in this area there is a colossal amount of negative material from the field of traumatization, e.g., for ritual reasons (circumcision in Jews, foot binding in Chinese women). Damages by serological means were produced by Guyer and Smith (1908–1924). They injected a pulverized rabbit lens into the blood of a chicken; a few days later, the serum of such a chicken was injected into pregnant rabbits on the 10th-13th day of pregnancy, when the development of the embryonic lens takes place. In the offspring of such rabbits, baby rabbits with lens opacity, microphthalmia, and other eye defects were partially discovered. In subsequent generations, the inheritance of these defects was discovered. The numerical ratios observed were incorrect. The authors interpret their experiments as the influence of an antibody produced in the chicken's blood on the rabbit embryos by the type of somatic induction. However, it is very surprising in such a case that the pregnant females themselves did not show the effect of the supposed antibody—neither their eyes nor their germ cells suffered. In addition, it is known that eye defects in the embryo are a highly nonspecific reaction and can be obtained as a result of the action of very different agents (alcohol, naphthalene feeding, radium, X-rays, etc.). Finally, verification experiments by Finlay (1924), Huxley and Carr-Saunders (1924) did not confirm the hereditary transmission. Therefore, at present Guyer and Smith's data are not recognized as conclusive.

The inheritance of psychological characteristics includes recent experiments by I. P. Pavlov (1923). He claimed that he succeeded in hereditarily fixing a conditioned reflex of response by sound to food in mice: the formation of a conditioned reflex with each generation required less and less training. However, experiments on mouse training by MacDowell (1924) and Vicari (1924) yielded a completely negative result. Finally, Pavlov himself (1927), with improved methodology and control, did not obtain confirmation of the initial experiments.

Changes caused by the external environment. A large number of attempts have been made to obtain hereditary changes under the influence of environmental variables. In all these experiments, the influence of the environment (temperature, light, humidity, etc.) is so universal that the scheme of somatic induction is entirely inapplicable to them. The experiments of Standfuss (1897) and Fischer (1901) on changes in pigmentation in butterflies gained the greatest fame. By exposing pupae of the small tortoiseshell (Vanessa) and the tiger moth (Arctia) to lowered temperatures, butterflies with coloration strongly darkened compared to the norm (so-called aberrations) were obtained in a small percentage of cases. When the altered butterflies were crossed with each other, a certain percentage of butterflies modified in the same way as the parents was obtained in the offspring, although their pupae had already been kept at normal temperatures. However, selection of hereditary coloration factors already present in the butterflies may have played a role here. After all, only an insignificant percentage of butterflies proved susceptible to temperature influence at the pupal stage, obviously having a hereditary potency for this. Indeed, Federley's experiments (1920, 1921) proved the presence of a large number of hereditary coloration factors in butterflies. At the time, great importance was attached to Tower's experiments (1906). Tower studied the effect of temperature on the Colorado potato beetle (Leptinotarsa). It turned out that influence on the egg and larval stages did not cause color changes in the beetles. Influence on the pupal stage causes a color change in the beetle that turns out to be non-hereditary. If, however, the beetle was subjected to influence at the moment of maturation of its germ cells, then although it did not change pigmentation itself, its offspring exhibited such a change, which was subsequently inherited. Thus, the change here occurred through direct influence on the germ cells during the period of their maturation, which was therefore called the "sensitive period." Recently, strong distrust has arisen regarding Tower's experiments, and they are not cited in many major manuals on heredity. Unfortunately, however, no one has repeated his experiments. In any case, they relate rather to the problem of idiokinesis and cannot be regarded as proof of the heredity of acquired characters. Of Kammerer's numerous works, the greatest attention was attracted by his work on changing pigmentation in the spotted salamander under the influence of the color of the substrate of its habitat (1913). Rearing the salamander on a yellow substrate and in a moist environment quantitatively and qualitatively enhances the yellow coloration; rearing on a black substrate and in a dry environment enhances the black coloration. Kammerer obtained offspring from such altered forms which, even when reared under normal conditions, proved correspondingly yellower or blacker. This work provoked extremely strong controversy and a series of verification works, with disagreements existing even in the interpretation of the latter. Thus, the thorough verification work of Herbst (1919, 1924) is considered by some to refute Kammerer's data, and by others not. Herbst found that changing the salamander's coloration under the influence of the background is possible only at the larval stage; in the adult state, not only is it impossible, but a reversion of the previously altered forms to the norm even occurs. In any case, without clarifying many questions (normal variability of the salamander, hereditary pigmentation factors, the role of selection, etc.), Kammerer's work cannot be regarded as conclusive. Long-term modifications. Experiments on the influence of environmental conditions on protozoa (Protozoa) revealed a certain peculiarity of the latter, explained by the specific features of their structure and modes of reproduction. However, despite these differences, the experiments established a phenomenon important for understanding the problem of the heredity of acquired characters in higher organisms. This phenomenon is called long-term modification. This concept was established by Jollos (1913–24) based on work on the influence of various external agents on infusoria. Thus, infusoria were subjected to adaptation to arsenic acid. Those surviving a lethal dose of it then multiplied under normal conditions and proved to have retained resistance to arsenic acid. In other experiments, infusoria were subjected to the prolonged influence of calcium ions, after which under normal conditions they exhibited a division rate approximately twice as slow as normal. However, detailed observation subsequently clarified that these acquired abilities sooner or later disappeared. With some exposures, the disappearance of the long-term modification occurs only after several divisions of the infusorium and immediately after parthenogenesis (endomixis) and conjugation. In other cases, long-term modifications can persist even after parthenogenesis and conjugation; then they disappear during further divisions. Depending on the conditions of extinction of long-term modifications, assumptions are made about their localization. It is hypothesized that long-term modifications depend on changes occurring either in the plasma or in the trophic nucleus of the protozoa (macronucleus). Without affecting hereditary factors, a long-term modification thus cannot be inherited and persists only as long as its material bearer (plasma, macronucleus) persists (on the inheritance of the results of use and disuse of organs, see Lamarckism). Criterion for the inheritance of acquired characters. The doctrine of pure lines and the phenomenon of long-term modifications show how strictly one should treat the proof of the inheritance of acquired characters. Experiments on the inheritance of acquired characters do not satisfy the requirements set by modern science on heredity—genetics. The basic requirements are: 1. Preliminary clarification of the hereditary content of the experimental material regarding the studied traits; work with a pure line or at least one homozygous for traits close to those being studied. 2. Elimination or accounting for the role of conscious and unconscious selection in the obtained results. 3. The inheritance of the acquired trait must be studied over a number of generations (at least three), and the regularity of inheritance must be established by crossing with an unconditionally normal control form. In the event of the emergence of a true hereditary factor (gene, see below), all the regularities established by modern genetics must be observed. As a result of reviewing data on the inheritance of acquired characters, we arrive at their complete and categorical denial. The recognition of this or that scheme for the inheritance of acquired characters has had, and partially still has, a reactionary and pernicious influence on the development of ideas and data of heredity in the fields of agriculture and medicine. The struggle against Lamarckism and its specific views in the field of heredity is a struggle against mechanistic and metaphysical concepts in favor of dialectical materialist biology and its application in agriculture and medicine. Modern foundations of the doctrine of heredity. Chromosomal theory of heredity. At present, a special science—genetics (see)—is devoted to the study of heredity. It is based on the regularities of heredity established by Mendel (see Mendelism). These regularities are observed during hybridization, i.e., the crossing of forms differing in one or another hereditary trait. Based on Mendel's laws, it turns out to be possible to consider the traits of an organism to a certain extent separately and independently. The behavior of observed traits upon crossing in the offspring makes it possible to draw a conclusion about the hereditary factor determining one or another trait. At present, the name proposed by Johannsen—gene (see)—has been adopted for the hereditary factor. Although Johannsen considered the gene to be a hypothetical concept, at present it should be considered a completely real material unit to which the regularities of heredity properly belong. Conversely, the concept of a hereditary trait is distinguished by a certain uncertainty and conventionality. A trait is the result of the development of an organism proceeding under certain environmental influences. Therefore, strictly speaking, not only the gene (or genes) participates in the realization of a specific trait, but also completely specific conditions of development, which may be different in each case. Thus, the Chinese primrose plant (Primula sinensis) possesses genes that determine flower color—red and white. However, it turns out that the trait of red flowers, which is known to be hereditary, manifests itself only at normal temperatures, whereas at elevated temperatures the flowers of the "red" race turn out to be white. This in no way means that these white flowers are equivalent to those of the white race and are caused by identical genes. By transplanting the "red" race back to normal temperatures, we will observe that newly blooming flowers on the same plant will be red. We will not achieve this from the white race. Thus, if the concept of a trait is characterized by conventionality, the hereditary factor determining it—the gene—on the contrary, is a completely definite, real hereditary unit.

In view of this, some geneticists introduced the concept of the "norm of reaction"—the gene determines the norm of reaction of the organism to a wide variety of conditions during the development of a given trait; in the example given, the trait of flower color. To characterize the relationship between genes and traits in modern genetics, the terms introduced by Johansen are accepted. The totality of hereditary factors (genes) of an organism is its genotype (see). (In Siemens's terminology, the genotype corresponds to the idiotype.) The totality of manifested traits of an organism is its phenotype (see). Thus, one or another hereditary trait is merely the phenotypic manifestations of the genotype determined for given conditions. Accordingly, one can speak of a specific phen (= phenotype) of a certain gene: the flower color gene of the "red" race at normal temperature causes a red color, at elevated temperature—white; while the color gene of the "white" race at any temperature produces a white color. Therefore, a change in the phenotype by no means signifies a change in the genotype: the gene for "red" flower color at elevated temperature does not turn into a gene for "white" color. The concept of a hereditary trait is further complicated by the circumstance that under certain conditions it may not appear at all. Thus, in the fly Drosophila, the gene causing doubling and splitting of the legs manifests itself depending on temperature: when developing at normal temperature, the percentage of individuals manifesting the gene is much smaller than at lowered temperature. Therefore, it is customary to speak of the degree or percentage of phenotypic manifestation of a gene under certain conditions. Again, this does not mean that in the given example the gene disappears or changes at normal temperature or arises at lowered temperature. The views that existed at one time regarding the inconstancy of the gene are presently untenable. The problem of the relationship between trait and gene is further complicated by the question of the number of genes causing the development of a trait and the number of traits determined by a gene. The point of view of Mendel and the early geneticists that one gene corresponds to one trait is naive and outdated. There are traits determined by several genes. Thus, the black skin color of Negroes is determined by several genes, and their action is identical: the more of these genes, the more intense the color will be. Such identical genes determining quantitative hereditary traits (intensity of color, height, etc.) are called equivalent factors (by other terminology—multiple factors) (see Polymeria). However, not only quantitative traits, but also qualitative ones can be determined by more than one gene. Thus, one or another coloration in rodents depends on the presence of at least two genes: one causing the pigment-forming principle—chromogen, and the other—determining the process of pigment formation—the activator (see Epistasis). Finally, besides such specific multiple factors determining the development of a single trait, there are many nonspecifically influencing genes. In general, it should be thought that each trait develops under the influence of the entire genotype. Thus, each gene, being the main determining condition for one or several traits, exerts to one degree or another a modifying influence on all other traits of the organism. Such an influence of a single gene on several traits is called pleiotropy (see). All those complications that the concept of a hereditary trait has undergone do not at all deprive the laws of Mendelism of their validity. The main provisions of Mendelism are the allelomorphism of genes, the phenomena of dominance and recessiveness, and the phenomenon of segregation (for details see Mendelism). At present, it is an indisputable proposition, first stated by Sutton (1902), that the regularities of Mendelism are explained by cytological processes occurring during the maturation of germ cells and fertilization. An organism is formed from a zygote (see)—the product of the fusion of two gametes (see)—an egg and a sperm. The most essential moment of zygote formation is the restoration of the normal number of chromosomes (see) characteristic of a given species. The theory of the individuality of chromosomes and the constancy of their number for each species, first put forward by Rabl (1885) and Boveri (1887), is presently completely proven and generally accepted. According to this theory, each species is characterized by a specific number of chromosomes and a specific shape of the latter. In the cells of the zygote and the adult organism, the chromosomes are pairwise identical, because they originated from the joining of two identical complexes (sets) of chromosomes in the gametes. The chromosome set of gametes, where each chromosome of individual form is present only in a single quantity, is called haploid. Two haploid sets form a diploid complex, where each chromosome form is present in a double quantity. Consequently, the organism arising from crossing has in all its cells a haploid set of chromosomes from the father and a corresponding set from the mother. From this follows the phenomenon of allelomorphism. If we assume that the gene determining one or another trait of one parent is localized in some chromosome introduced by the latter, then a similar gene must be located in the corresponding (homologous) chromosome from the other parent. Thus, all genes are pairwise allelomorphic. The phenomena of dominance and recessiveness represent the result of the relationship between two allelomorphic genes among themselves. When an organism is formed from a zygote and genes determine its development (how this happens is currently unknown), the allelomorphic genes turn out to be either antagonistic or their effect is summed up. In the first case, the allelomorphic gene completely suppresses or hides the potency of the other. This phenomenon of suppression is called dominance (see). The dominant gene is opposed by the recessive gene—the suppressed one (see Recessive). However, the phenomenon of incomplete dominance or semi-dominance is often observed, when the recessive gene has the opportunity to a greater or lesser extent to manifest its potency. Then the so-called intermediate heredity is observed, which was previously erroneously considered as refuting Mendelism. The causes determining the relationships of allelomorphs are largely unknown, and it is possible that the difference between dominance and recessiveness is conditional and relative. The phenomenon of segregation will be observed in the next generation. It is explained by the fact that the germ cells—gametes—of the previous generation possess a haploid set of chromosomes thanks to the reduction division (see) that occurred during their maturation. At the same time, it is equally probable that out of each pair of homologous chromosomes either one or the other will remain in the gamete. According to this same law of chance, the distribution of allelomorphic genes takes place. The combination of gametes during crossing also occurs in all possible relations. Thanks to this, in a certain proportion, recessive genes meet with each other and they manifest the recessive trait that remained hidden in the previous generation (for details see Mendelism). This parallelism of the regularities of Mendelism and chromosome distribution revived with new force the chromosome theory, definitely stated first in the form of the nuclear theory of heredity back in 1884 by O. Hertwig and Strasburger and supported and developed by Weismann and Boveri. Other brilliant proofs in favor of it came almost simultaneously from the field of sex determination. Sex determination. The diploid chromosome complex characteristic of each species of plants and animals usually contains an even number of chromosomes, because it is formed from two identical haploid complexes of gametes. Therefore, the chromosome complexes of the male and female of the same species should seem to be completely similar to each other in both the number and shape of chromosomes. However, it turns out that in numerous species this is not confirmed. At present, a very large number of species are known in which both sexes are characterized by unequal chromosome complexes both in terms of number and in terms of the shape of certain chromosomes. Cases are distinguished where the complexes of the male and female differ in the number of chromosomes; in the vast majority of such cases, the complex of one sex is smaller by one chromosome. In other cases, although the complexes of the male and female are numerically identical, one of the chromosomes in one sex differs in shape from its paired chromosome. In all these cases, the sex having the unpaired chromosome in shape or a smaller number of them produces two kinds of gametes. The sex producing two kinds of gametes in a given species is called digametic, or heterogametic; then the other will be monogametic, or homogametic. McClung (1902) and especially Wilson (1905 and later), based on data on the dimorphism of chromosome complexes, developed the theory of sex determination, which is now generally accepted. At present, autosomos and heterosomes or X-chromosomes are distinguished in each complex. Autosomes are called chromosomes that are present in equal numbers and form completely homologous pairs in shape in both the male and the female. In addition to autosomes, in a multitude of species, chromosomes are observed that either have no paired partner at all or the latter is not homologous in shape.

Thus, in the insect Protenor, the female has 7 pairs of chromosomes in her body cells, while the male has only 6 pairs and one unpaired chromosome. In this insect, therefore, the male is digametic and produces two kinds of gametes: with seven and with six unpaired chromosomes. The female is monogametic and produces all gametes identical with seven unpaired chromosomes. In the case of the fusion of gametes carrying seven chromosomes each, a zygote with seven pairs of chromosomes will be obtained. From such a zygote a female will develop, which will again be monogametic. In the case of the fusion of gametes with six and seven chromosomes, a zygote with 6 paired and 1 unpaired chromosome will be obtained. The zygote will develop into a digametic male; the X-chromosome is the name given to the chromosome that distinguishes the gametes for the male from the gametes for the female. Consequently, in the male, the X-chromosome is single and unpaired; in the female, they form a pair. In view of this, this type of sex determination is schematically depicted for the male as X-0 (X, zero), and for the female as XX (two X-chromosomes). Essentially, sex determination in the moth Abraxas does not differ at all from this type, sometimes called the Protenor type. The only difference is that in the latter, it is not the male, but the female sex that is digametic. Therefore, the Abraxas type is as follows: male XX, female X-0. There exists another type of digametic condition, referred to as the Lygaeus type. In the insect Lygaeus, both the male and the female possess 7 pairs of chromosomes. However, in females, the partners in each pair are completely similar to each other, whereas in males, in one pair, the chromosomes are sharply dissimilar: one is the same as the chromosomes from the corresponding pair of the female complex, while the other is considerably smaller in size. Consequently, in Lygaeus, the male is digametic, and the Lygaeus type will be male XY, female XX (Y denoting the dissimilar heterochromosome). This type is also observed in female digamety. Thus, in the moth Phragmatobia, X- and Y-chromosomes are also present in the female. The Phragmatobia type will be: male XX, female XY. Thus, X-chromosomes and Y-chromosomes, or heterochromosomes, determine the sex of the zygote, and therefore they are generally called sex chromosomes. Although all the described types were first established in insects, they are common to all species. In those apparently less widespread cases where morphologically all pairs of chromosomes of the complex are indistinguishable from one another, it is believed that digamety nevertheless takes place in such species, i.e., it should be considered that their X- and Y-chromosomes are only morphologically homologous. Male digamety is observed, besides insects, in mammals, including man (see below). Female digamety has been established in birds. Parallel with the cytological study of the problem of sex determination, a number of attempts were made to establish the inheritance of sex. Correns drew attention to the fact that the usually observed numerical sex ratios—1:1—are completely similar to the numerical proportions of "test crossing," i.e., when a recessive homozygous form is crossed with a heterozygous one (see Mendelism). On this basis, he concluded that one sex is heterozygous for the factor determining it. Subsequently, complete correspondence of the "heterozygosity" of one sex with its "digamety" was revealed. This proved to be an extremely convincing argument in favor of the chromosome theory of heredity. Morganism. However, decisive experimental proofs of the chromosome theory of heredity were obtained predominantly by the works of T. H. Morgan and his coworkers. In 1909, Morgan undertook experiments with the fruit fly Drosophila melanogaster to obtain mutations in it under the influence of various substances, food, and radium rays. [As an object for experimental work, Drosophila was first used by Castle (1902).] Having obtained no results, Morgan took up the selection of certain traits. Selection also gave no results, but at the same time he managed to discover a number of mutant forms. Already in that same year of 1910, he discovered in a whole series of lines a multitude of mutations, the study of the inheritance of which he then undertook. The first regularity that he managed to establish in this connection was so-called sex-linked heredity (first described, however, by Doncaster in 1906 in the moth Abraxas). It turned out that the regularities of sex-linked heredity completely coincide with the regularities of the distribution of sex chromosomes, and therefore it is natural to conclude that factors inherited in such a manner are localized in the sex chromosomes (for details see Sex). A multitude of other mutations studied by Morgan's school were inherited independently of the sex, and on this basis they had to be localized in the autosomes. Subsequently, the observation of the inheritance of two or more genes localized simultaneously in some chromosome (first observed for genes of the sex chromosome) led Morgan and his coworkers to the formulation of the theory which is properly sometimes called morganism. In its completed, modern form, morganism includes the following generalizations: the theory of the number of linkage groups, the theory of the linear arrangement of genes, and the theory of crossing-over. All these theories were created to explain the numerous facts of apparent violation of numerical Mendelian relations (see Mendelism, Genetic analysis). According to the views of Morgan's school, now generally accepted, genes are localized in all chromosomes of the complex inherent in a given species. If we observe the inheritance of genes located in different and non-homologous chromosomes, their distribution will occur according to the law of chance, i.e., regular Mendelian numerical relations will be observed. In the case of the inheritance of genes located in the same homologous chromosomes, phenomena of so-called linkage or repulsion will be observed, as a result of which unusual numerical relations of genotypes and phenotypes will be obtained (for details see Chromosome crossover). The phenomenon of linkage and repulsion was described earlier by Bateson and Punnett (1906) under the name of gametic correlation. Since linkage is always observed between certain genes, these latter are called a linkage group, and it is considered that this entire group is localized in homologous chromosomes. Consequently, the genes observed in any species can form as many linkage groups as the complex of the given species has pairs of homologous chromosomes. This constitutes the theory of the number of linkage groups, which can presently be called a law, since not a single violation of it is known. Moreover, if apparent violations of it were observed, it turned out that in these cases a change in the number of chromosomes, and consequently in the number of linkage groups, had taken place. Genes forming a linkage group, i.e., localized in one chromosome, exhibit varying degrees of linkage among themselves. The degree or strength of linkage between two specific genes is a more or less constant value and is characterized by the numerical relations observed during their hereditary distribution in genotypes and phenotypes (for details see Chromosome crossover). To explain the various degrees of linkage between genes, Morgan proposed (1911) the theory of the linear arrangement of genes. The rudiments of this theory are encountered, as mentioned above, in W. Roux (1883) and in Correns (1902). In a more completed form, it was purely hypothetically formulated by de Vries (1903) and Strasburger (1908, 1909). According to this theory, genes are located linearly along the length of chromosomes at a certain distance from each other. The violation of linkage between genes occurs due to the phenomenon of crossing-over, or chromosome crossover. In creating the theory of crossing-over, Morgan utilized the cytological theory of chiasmatypy of Janssens (1909). It is assumed that homologous chromosomes at a certain stage of maturation of reproductive products touch and intertwine with each other (see Reduction division, Chromosomes). During this intertwining of chromosomes, they can exchange their homologous parts. Two or more genes that were located in the same chromosome can thereby end up in different homologous chromosomes. This phenomenon of gene exchange between homologous chromosomes bears the name of crossing-over. The probability of crossing-over between different genes is completely definite and is expressed numerically. It is assumed that this probability is a function of the distance between genes in the chromosome, namely: the greater the distance, the higher the frequency of crossing-over. Based on the frequencies of crossing-over between various genes obtained in crossing, Sturtevant (1913 and later) constructed a map or chart of chromosomes, i.e., he expressed linearly the relative distances between genes in a given chromosome. At the present time, chromosome maps are known only for the most studied forms, such as, for example, the fruit fly Drosophila, corn, chicken, grasshopper, and others (see Chromosome crossover). If at present we possess significant knowledge about the mechanism and regularities of heredity, much less is known about what the hereditary substance represents, in what manner it realizes its properties during the development of the organism, and what the regularities of the changes it undergoes are. According to modern views, the hereditary substance localized in the chromosomes can undergo a whole series of dynamic processes that cause changes in hereditary traits.

In general, any change in the hereditary substance is called a mutation (see Mutation). Types of mutations are distinguished depending on which material unit determining heredity is altered. The process associated with the origin or alteration of a gene is called transgenation, or point mutation, because it is believed that a gene is an extremely small region of a chromosome. One and the same gene can yield a series of transgenations causing very diverse changes in the trait determined by the given gene. All these transgenations will be allelomorphic to one another, that is, being paired in the same genotype, they will exhibit phenomena of dominance, recessiveness, or intermediate manifestation among themselves. Such a group of allelomorphic genes bears the name of multiple allelomorphs. However, changes occurring in entire regions of a chromosome are frequently observed. The loss or inactivation of a chromosomal region is called deficiency, or a shortage. The region that has undergone loss or inactivation does not exhibit the properties of the genes that were localized in it. Therefore, the recessive genes of the same region in another normal chromosome are manifested in this case in the heterozygous state. Moreover, an organism with a deficiency in the homozygous state cannot exist, because the zygote turns out to be lethal, that is, non-viable (see Lethal genes). Recently, cases of the loss of a very large middle region of a chromosome have been observed, in which the remaining insignificant ends of it, by joining, form a very small chromosome. Such a strong deficiency is called a deletion. If, upon the loss of a chromosomal region, it attaches to another chromosome, this phenomenon is called translocation. With the translocation of a region, the genes localized in it join the linkage group to which all the genes of the new chromosome carrying the translocated region belong. The displacement of a region within the same chromosome, usually its rotation by 180°, is called inversion and typically causes the cessation of crossing-over along its length. All those phenomena that occur with individual regions can also be observed with respect to entire chromosomes. Thus, the loss of entire chromosomes bears the name of elimination. The elimination of one or another chromosome at one or another stage of development explains the phenomena of mosaic heredity, gynandromorphism, and the like. The attachment of a chromosome as a whole to another bears the name of association or pairing of chromosomes. In this case, two gene linkage groups form one common group. The process inverse to association, in which a chromosome breaks down into two or more, is called fragmentation. Finally, cases are observed when some chromosome turns out to be in an excess number—instead of two homologous chromosomes, there are three, four, and so on. This phenomenon bears the name of polysomy and usually takes place as a result of the crossing of two forms with an unequal number of chromosomes. A particular case of polysomy is chromosome nondisjunction. Nondisjunction of sex chromosomes, discovered in Drosophila by Bridges (1913), amounts to the fact that during the formation of gametes, two X-chromosomes fall into one of them, and not a single one falls into the other. The hypothesis of nondisjunction, created to explain violations in the inheritance of sex-linked traits, was fully confirmed by cytological analysis and serves as one of the striking proofs of the chromosome theory of heredity. However, hybridization is not required for nondisjunction, as it is for typical polysomy: it is explained by an improperly occurring reduction division (see), in which the chromosomes are distributed unevenly. In nondisjunction and polysomy, complex interrelationships of allelomorphic genes and peculiar heredity are observed. Finally, changes affecting not individual chromosomes, but the entire complex as a whole, belong to the phenomena of polyploidy. In polyploidy, the haploid complex turns out to be, instead of the usually doubled one, tripled (triploidy), quadrupled (tetraploidy), and so on. Polyploidy exerts a very complex effect on traits. As diverse as the nature of changes in the hereditary apparatus is, so diverse are, apparently, the causes of these changes. The phenomena of polyploidy and polysomy are apparently largely caused by the influences of external conditions (drugs, humidity, temperature, and the like), as well as by hybridization. As for transgenations, all attempts to experimentally induce them by the action of external factors had been unsuccessful until very recently. Only in the very latest years has it been possible to discover external agents causing transgenation. Such agents are X-rays and radium rays. Gager and Blakeslee (1927) on the Jimson weed plant and especially Muller (H. J. Muller; 1927) on the fruit fly Drosophila achieved the production of transgenations by means of the action of radium and X-rays. X-rays induce both transgenations and translocations, deletions, and so on. Further, apparently the action of ultraviolet rays (Altenburg; 1931) also induces the mutation process. Finally, Goldschmidt (1929) and Polozov (1930) assert that by the influence of elevated temperature on Drosophila larvae they managed to induce mutations. However, these data must be treated with caution until it is proven that in this case we are dealing with the same gene changes as in the mutation process. The discoveries of Morgan's school played a decisive role in the concretization of conceptions of the gene. Morgan even believes that a "theory of the gene" can already be formulated. Indeed, the basic regularities experimentally established by Morgan's school are indisputable and cannot be characterized as "formal," because they are real, objective regularities. However, in modern morganism there are tendencies toward certain erroneous, metaphysical, theoretical conceptions that have roots in weismannism (see above). Thus, Morgan, having begun with attempts to induce mutations by external factors, subsequently became more and more entrenched in the conviction of the exclusively autogenetic character of the causes of gene change. In 1926 he wrote: "It is difficult to imagine any influence from the environment that could be the cause of the change of one gene in a cell without affecting the other identical (allelomorphic) gene in the same cell. Hence it seems more probable that the cause of change is internal rather than external." However, Muller's discovery in 1927 completely refuted this argumentation. An even more serious methodological error is committed by Morgan on the question of the origin of genes. He says: "At present there is no data that would allow one to think that new genes arise in any way other than through changes in the structure of old genes. The total number of genes remains on the whole constant over long periods" ("The Theory of the Gene," p. 88). However, Morgan does not stop at this cautious empirical formulation and in another place distinctly declares: "there seems to be no need to assume a smaller number of genes for the amoeba and a larger one for man" (p. 276). This formulation differs very little from the views of Lotsy *, categorically rejected by modern biology (see Evolutionary doctrines). Thus, Morgan comes to a denial of the historicity of the gene. If we consider it necessary to admit for the 90 modern chemical elements a single origin through the complication and development of some basic initial form, then all the more we cannot admit the eternity of many thousands or in any case hundreds of modern genes **. These and some other erroneous conceptions of Morgan and other individual representatives of morganism cannot, of course, discredit the main achievements of genetics, in which the same Morgan, as we have seen, played a significant role. * Lotsy asserts not only the constancy of the number of genes, but also denies any mutational changes in them. Morgan, on the other hand, as we have seen, recognizes gene changes, albeit for internal reasons. ** The problem of the origin of genes should not be confused with the problem of changes in existing genes, that is, transgenations. Nature of the gene. Regarding the nature of the gene at the present time, there are only hypothetical conceptions. The earliest theory of the gene is the "presence-absence" theory of Bateson and Punnett (1905). According to this theory, the origin of new recessive genes is due to the mechanical loss of dominant genes. However, at present this theory has been abandoned, because it completely contradicts the diverse qualitative changes and directions of the transgenation process. Thus, first of all, it is known that the concepts of dominance and recessiveness are relative and therefore in cases where the heterozygous form is "intermediate," the participation of the recessive gene in the realization of the trait must be admitted, which could not be if it were simply an "absence." Another objection relates to the phenomenon of so-called reverse mutations. A mutation from a recessive gene to a dominant one, often causing a "normal," "wild" trait, is called reverse. A number of reliable cases of reverse mutation have been described; furthermore, recently it has turned out that they occur relatively frequently under the influence of X-rays.

Finally, the third group of objections, which played the greatest role in discrediting this theory, relates to the phenomenon of multiple allelomorphism and was argued by Morgan and his coworkers. The phenomenon of multiple allelomorphism shows that a gene can change not only in one direction—recessive or dominant—but is capable of producing the most diverse qualitative changes in any direction. Thus, in Drosophila, a gene localized at point 1.5 of the X-chromosome and responsible for eye color gave rise at different times and in different lines to 11 forms of mutational changes causing the replacement of the usual red eye color by white, ecru, slightly colored, pale yellow, ivory, eosin, apricot, cherry, blood-red, coral, and wine. Thus, there can be many more allelorphic genes than two, and they exhibit a whole gamut of degrees of dominance and recessiveness. The "presence-absence" theory is unable, without additional complex assumptions, to explain the phenomenon of multiple allelomorphism, since it admits only two states of a gene—its presence or absence. The mechanistic nature of the "presence-absence" theory is obvious. Most geneticists view the transgenation process as associated with qualitative chemical changes of the gene. It should be thought that the gene is a material particle: according to some, a single molecule, while according to others, more than one, consisting of nucleoproteins. According to some views, genes are localized in the staining substance of the chromosome—basichromatin; however, the view that they are enclosed in skeletal or lenin axial formations of the chromosomes—oxychromatin—seems more probable (see Chromosomes). The nucleoprotein molecules of which genes are supposedly composed must possess such a complex chemical structure that the number of their possible isomers is practically immeasurable. The transgenation process may well consist in stereochemical changes of gene molecules. As for the process of so-called hereditary realization, i.e., the regularities by which the influence and determination by genes of the development of the organism and its characters are realized, almost nothing is currently known about this. The enzyme theory of the gene, created by Hagedoorn (1911), according to which genes are autocatalysts, is the most widespread. However, it would apparently be more correct to admit that while some genes represent catalysts of certain developmental processes (e.g., pigment formation), others may participate in development not as activators, but as a material substrate (e.g., chromogen in pigment formation). It remains unclear whether genes act on the cytoplasm always or only during certain periods of development. The boundaries of Mendelian heredity. At present, it can be considered firmly established that heredity determined by factors localized in chromosomes obeys the regularities of Mendelism. Therefore, so-called non-Mendelian heredity should be sought outside the chromosomal apparatus. If this or that form, in addition to the chromosomal complex, had structural elements of the cell possessing constancy and genealogical continuity, the characters determined by these elements could be inherited. An example of such elements is plant plastids. Correns (1907) described a case of hereditary transmission of variegation in plants, in which the offspring characters were determined entirely by the maternal plant. This phenomenon was explained by the transmission through the egg of plastids determining the nature of chlorophyll development. Later Baur (1909) discovered cases in which plastids were transmitted not only by the egg, but also by pollen. In plastid heredity, exact numerical regularities are not observed, since the distribution of plastids apparently occurs irregularly. Moreover, at present the question of the permanency of plastid formations is controversial. In the case where plastids are formed anew, plastid heredity must be of a sporadic and short-lived character. Finally, the number of characters determined by plastids is very limited. At the same time, the influence of genes on the activity of plastids is precisely established. Formations analogous to plastids in animal cells are chondriosomes (see), or plastosomes. Meves (Meves; 1908) put forward the theory of chondriosomes as carriers of heredity. He asserted that chondriosomes are permanent carriers of plasma properties and objected to the "hereditary monopoly of the nucleus." However, this theory did not meet with support, because it did not put forward any distinct factual evidence in its favor. At one time it was believed that in interspecific crosses a special heredity is observed, so-called constantly intermediate. However, on the one hand, in many interspecific crosses it was possible to establish Mendelian segregation, while on the other hand, it was discovered in a number of interspecific crosses that the absence of segregation is explained by violations of the normal mechanism of chromosome distribution. Of great interest are observations on the interrelationship of plasma and genes in interspecific and intergeneric crosses. For example, Wettstein (Wettstein), when crossing different species of mosses, discovered the influence of the plasma of a given species on the realization of genes. On this basis, he established the concept of the plasmon, by which he means the specific features of the plasma of a given species. Apparently, this peculiar antagonism of plasma and genes is explained by the absence of stable interaction when genes are introduced into a plasma alien to them. Thus, factual data on the existence of a specific carrier of heredity besides genes do not exist. However, there is a view, stemming from theoretical considerations, that Mendelian heredity relates only to characters distinguishing races from one another, while the main features of the organization of the organism common to many species, genera, classes, and even types are not subject to Mendelian heredity (Boveri, Conklin, Loeb, Plate, and others). In any case, at present no special non-Mendelian heredity is known to us. Human heredity—see Man.