Polymeria

POLYMERIA. 1) P. in genetics—a term proposed by A. Lang (1910) for phenomena of heredity in which different degrees of expression of identical inherited traits are determined by the quantity of genes with identical effects. The first clear cases of P. were thoroughly studied by the Swedish geneticist H. Nilsson-Ehle (1909 and later) in oats and wheat. The black color of oat seed husks and the red color of wheat ears and seeds, being dominant traits, in a number of cases instead of the usual segregation of 3:1 showed segregation in the ratio of 15:1 and 63:1; moreover, the coloration of dominant forms showed variation in intensity. Nilsson-Ehle made the assumption: the coloration depends not on one pair of genes, but on two, three, or more, and it should be assumed that each dominant gene of a specific coloration has an identical effect. Thus, in the segregation in wheat into 15 red plants and 1 white, it is assumed that the red color is caused by one dominant gene, as well as by two, and the plants expected in dihybrid crossing with two dominant genes are similar in coloration to plants carrying one or the other dominant gene separately. However, this similarity is incomplete, since the intensity of coloration turns out to depend on the number of dominant genes present. Such identical hereditary factors have been called univalent factors (in another terminology—multiple factors), and the explanation of the phenomena of P.—the principle of Nilsson-Ehle. However, Gregor Mendel (1865) had already assumed to explain the diversity of shades of flower and seed coloration observed by him in bean crossings (Phaseolus) the presence not of one gene for coloration, but of two or three; he also foresaw the resulting from this assumption change in numerical segregation ratios to 15:1 and 63:1. The principle of P. is an extremely important generalization in theoretical and practical respects. The theoretical significance of the principle of P. lies in explaining phenomena of intermediate and constantly intermediate heredity. Thus, it was assumed that mulattoes are a constantly intermediate hybrid form between white and black races and, contrary to the rules of Mendelism, supposedly no segregation and reappearance of the white recessive form is observed in them. However, Davenport (1913) showed that segregation is actually observed, but in ratios close to 15:1, i.e., one should assume the presence of two pairs of genes for skin pigmentation. Generally, in P. the following phenotypic regularities are observed among other things: 1. In the case of crossing a recessive form in all polymer genes with a fully dominant form, in F₁ offspring intermediate in phenotype is observed. 2. Phenotypically identical forms may have the same number of dominant polymer genes, however not homologous (e.g., AAA₁A₁ and A₂A₂A₃A₃), then in F₁ complete repetition of parental forms (AaA₁a₁A₂a₂A₃a₃) will be observed, and in F₂—unexpected range of variation. 3. If parental forms have an unequal number of partially or completely non-homologous dominant polymer genes (e.g., AAA₁A₁ and A₂A₂A₃A₃), then in F₂ variation of phenotypes will be observed, the range of fluctuation exceeding the parental forms, with smaller (AA and aa) and larger (AAA₁A₁A₂A₂A₃A₃) number of dominant genes. All these regularities have enormous practical significance in plant breeding and animal husbandry, since when crossing different races, accumulation of polymer genes of quantitative traits (growth, weight, productivity, intensity of coloration, etc.) that may have economic importance occurs. However, it should be kept in mind that the univalence of polymer factors should theoretically be considered as a conditional and relative concept. Thus, in the above example of P. of human skin pigmentation, there are actually at least two qualitatively different pigments; moreover, the thickness of the skin also affects the intensity of pigmentation. Besides, it should be kept in mind that the phenomenon of P. in one or another specific case may be determined simply by the pleiotropic effect of genes determining not identical traits at all (see Pleiotropy). A synonym for P. is homomeria (Plate). Siemens' term—polyidy—covers all cases of polygenic heredity, including P.

A. Gaisinovich. 2) P. in chemistry. The phenomenon of P. consists in that there exist substances, identical in composition, but having multiple molecular weights, genetically related to each other and differing from each other in physical and chemical properties. Such a definition indicates that P. occupies an intermediate position between the phenomenon of chemical association, in which only the physical properties of the substance change (connection between molecules through residual valences), and the phenomenon of chemical condensation (see Chemical condensation), which is characterized by the splitting off of atoms and formation of new chemical bonds. The phenomenon of P. is based on the mutual saturation of free valences of several unsaturated molecules (monomers), forming stable complex molecules (polymers) (see Polymerization). Only substances for which the genetic connection is proven by the realization of at least in part a reversible process: polymerization-depolymerization, can be considered connected by polymeria. The concept of P. was introduced into science by Berzelius after Faraday found in illuminating gas butylene, having the same elementary composition but twice the molecular weight than ethylene: 2C₂H₄ = C₄H₈. Since then, during a hundred years, numerous cases of P. of various classes of organic compounds have been studied. In recent years, interest in the phenomenon of P. has increased due to works on elucidating the structure of natural organic substances: rubber, cellulose, starch and other polysaccharides and proteins. The complexity of the phenomenon of P. required the introduction of new research methods. In the study of P., not only the usual methods of structural and physical chemistry are used, but also the latest methods of colloid chemistry (for example, X-ray photography of solid polymers). Several types of polymers are distinguished. Type I is intermediate between true polymers and the product of condensation. When two unsaturated molecules combine, migration of hydrogen atoms occurs; example: isobutylene and diisobutylene CH₃\CH₂ CH₃/CH₂ CH₃/C=CH₂ CH₃/CH₂ CH₃/C-CH₃ CH₃\CH₂ CH₃/C=CH₂ To this type belong polymers of nitriles, aldols, formation of complex esters from aldehydes. Type II. When two unsaturated molecules combine, a 4-membered ring is formed. This includes many cases of P. of ethylene derivatives, for example: 2C₆H₅CH=CHCOOH → C₆H₅CH-CHCOOH C₆H₅CH-CHCOOH cinnamic acid truxilic acid Type III. Three unsaturated molecules combine into a 6-membered ring. This is how P. of acetylene derivatives into aromatic compounds, aldehydes into paraldehydes, of hydrocyanic acid—into cyanamide and cyanuric acid, etc., proceeds. HC≡CH → C₆H₆ H-C≡C-H → H-C=C-H H-C=C-H acetylene benzene H-C=O H-C=O H-C-R H-C-R H-C-R H-C-R aldehyde H-C-R → O=CH-R O=CH-R O=CH-R paraldehyde Type IV. A large number of unsaturated molecules (often reaching several thousand) forms a chain; as a result, the unsaturated nature of the ends of the molecule hardly manifests itself with respect to the whole molecule (Staudinger). This macromolecule is identical with a colloidal particle, which under external influences, e.g., temperature, has different magnitude and consequently different physical properties; therefore, the determination of the molecular weight of these polymers leads to different results. Polymers of formaldehyde, styrene, butadienes, etc., have this type of structure. High-molecular natural polymers, such as rubber, cellulose, protein, belong to the same type. Type V. Compounds containing divalent carbon pass into a polymer with tetravalent carbon (methylene polymeria). To this type belongs the polymeria of hydrocyanic acid, H-C≡N, of fulminic acid C≡N-OH, and of isonitriles R-N=C.