VARIABILITY

From Big Medical Encyclopedia

VARIABILITY — property of organisms to change the morpho-physiological organization, the causing variety of individuals, populations, races etc. And. it is inherent in all organisms and it is observed even at genetically closely related individuals having similar or general living conditions and development, napr, twins, members of one family, strains of microorganisms and vegetatively breeding individuals. The changed form of the individual, population, race etc. received the generalized name of option.

And. any features of an organism, whether it be morfol are subject., fiziol, or biochemical, signs. And. can mention as quantitative characters, t. e, those which can be measured or counted (e.g., number of fingers, vertebrae, hair, weight and the sizes of a body), and qualitative (e.g., color of eyes and coloring of integuments). In a basis And. lies or change of reaction genotype (see) on factors of the environment, or change of the genotype as a result of mutations of genes (see. Gene ) and (or) chromosomes (see) or recombinations (see). In this regard the general (phenotypical) And. subdivide on hereditary and not hereditary.

Fig. 1. The diagrammatic representation of a water buttercup at modification variability of a form of surface and underwater leaves (are specified by shooters).

Not hereditary (environmental, paratypical, modification) variability is caused by influence inside - and extracellular factors on manifestation of a genotype. Environmental factors have significant effect on manifestation of many genes. So, e.g., if the person has hereditary predisposition to completeness, then on a limited diet it can keep the weight in normal limits. At the same time the people who do not have such predisposition, but not limiting themselves in food can gain the weight considerably exceeding norm. The main thesis of genetics says that a phenotype (see. Genotype ) represents result of interaction between a genotype and Wednesday. The genotype defines number of possible phenotypes under various conditions of the environment. Range of possible manifestations of a genotype depending on conditions of the environment is called a hereditary reaction norm. Change of this or that sign under the influence of conditions of the environment often fluctuates within environmental standard of reaction of this look and has character of adaptation or devices. Therefore this form I. call also modification. Therefore, heredity determines limits modification by I. Primer of such plasticity of a phenotype formation of leaves at a water buttercup (Ranunculus aquatilis) is, at to-rogo a form of surface leaves of the same plant strongly differs from underwater (fig. 1).

Hereditary (genotypic) variability makes that part of the general phenotypical variability, edges is defined by genetic distinctions between individuals or groups of individuals.

And. quantitative (metric) characters it is characterized by lack of jumps; it call gradual (quantitative, continuous). At quantitative And. between individuals as carriers of signs it is impossible to carry out accurate differentiation since they are connected with each other by a set of intermediate forms. Quantitative characters are controlled by a large number of genes which function as uniform system (so-called polygenes), and strongly vary under the influence of environmental factors. In such cases in firstgeneration of F1 the posterity has a transition type, and in F2 accurate Mendelian splitting on strictly different groups is not observed (see. Mendel laws ).

Discontinuous And. (qualitative, alternative, spasmodic) it is characteristic of qualitative characters. At qualitative And. individuals can be divided into accurately differentiated not blocked groups according to existence or lack of this sign.

Fig. 2. Sprouts of mutants albinos of barley among normal sprouts.

Inheritance of qualitative characters is controlled usually by genes of one or several loci which alleles possess the expressed individual manifestation and are distributed according to Mendel's laws. Typical Mendelian inheritance is characteristic of many signs of the person. In particular, one of such signs is albinism when there is practically no pigment in skin, hair and eyes. Albinism is found also in many animal species and plants (fig. 2). At the person about 30% of the studied loci make loci with nonidentical alleles (heterozygote). If to accept total number of structural genes at the person of equal 100 thousand then each individual can be to genes, heterozygous on average on 30 thousand, than and extremely high genetic polymorphism in populations is caused. E.g., individual distinctions in reactions to medicines are connected with polymorphism of the fermental systems regulating the speed of metabolism of this or that medicine and removal of products of its exchange from an organism.

Leading role in processes genotypic And. plays change of the reproduced structural elements of a kernel (DNA of chromosomes) and it is much less than reproduced elements of cytoplasm (a mitochondrion of animals and plants, a plastid of plants). Mutations (see. Mutation ) can be near are caused also a physical., chemical and biol, factors. Changes of a genotype lead to change of a developmental character of this or that sign in these conditions of the environment, change of a hereditary reaction norm and emergence of various deviations (hereditary anomalies of development). Along with mutations source genetic And. recombinations of genes are, so-called recombinational And. Genotypes — no other than combinations of genes. At a syngenesis there is an association of two haploid sets of the chromosomes formed of diploid sets of parent individuals (or cells), in numerous zygotes, each of which is almost unique. The number of possible combinations even for a small number of genes is huge. One couple of parents differing from each other only on alleles of 100 loci can give the size of combinations of genes equal of 2100. Biol, value of a syngenesis is that the producing various genotypes at such reproduction considerably accelerates. On degree genetic And. in population also the gene-spread from other populations influences. The factors reducing degree And., generally are natural selection (see) and accidental influences.

At modification And. hereditary structures of sex cells do not change; only change of manifestation of genes in somatic cells is observed that is expressed in external signs (phenotype) of an organism. If influence of environmental factors considerably exceeds borders in which existence of an organism is possible, or is caused by the agent not inherent to that Wednesday, to a cut there was an adaptive evolution of a look, then in such cases there is a disturbance of development, there are inborn malformations (uglinesses) or even death of an organism is possible. The environmental factors causing anomalies of development of a germ of the person or animals are called teratogens (see. Teratogenesis ). Physical concern to them. factors (X-ray, alpha, beta and gamma radiations, gravitational overloads, vibrations), chemical factors (toxic chemicals, medicinal substances, etc.) and biol, factors (some viruses, microbes, the elementary). For many teratogens direct dependence of types of inborn malformations on a stage of an embryogenesis is described, on a cut the germ was affected by this factor. Under the influence of various classes of teratogens there are identical changes of bodies, and these malformations are similar to the hereditary anomalies caused by mutant genes or aberation chromosomes. The uglinesses caused by teratogens which are similar to hereditary anomalies are called phenocopies (see. Genotype ).

Similarity of the inborn malformations resulting from action of disturbing factors of the environment and hereditary anomalies is explained by the fact that at mutations and under the influence of teratogens in cells of the forming fabrics and bodies the same links biochemical, processes are broken.

Methods of studying

Analysis of the nature And. and transfers of the changed signs from generation to generation serves as the main means of studying of the mechanism heredities (see). Externally indiscernible phenotypes in identical conditions of the environment in one cases can be result of change of a genotype, and in others — result of not hereditary modifications. Without genetic analysis (see) it is impossible to define what share phenotypical And. it is caused by not hereditary modifications and what genetic factors. If population consists of individuals with identical genotypes, then artificial selection (see) in such population it will be unpromising since all observable phenotypical And. it is caused by influence of the environment. Some lamarkist mistakenly believed that not genetic modifications, so-called acquired signs, can turn into hereditary changes. Such point of view was not confirmed and is of only historical interest now. This 60 — the 70th 20 century show that transformation of modifications into hereditary changes is absolutely impossible. In process of development geneticists (see) it became clear that opposition of mutations (large qualitative changes of signs) to fluctuations (small quantitative variations) is absolutely wrong. Mutations can mention any signs of an organism, at any stage of its development, and besides the most various taxonomical value. Analysis of the reasons and development of exact quantitative variation and statistical ways of assessment And. showed that mutations can be the cornerstone of insignificant quantitative deviations, externally not distinguishable from not hereditary modifications. For identification of a relative role of the environment and a genotype in formation of this or that sign use two main complementary methods: one of them consists in a research genotypic of various individuals in as much as possible identical environment, and another — in studying of genetically identical individuals in different conditions of the environment. It is easy to apply both methods to such organisms as bacteria and yeast which as a result of repeatedly repeating divisions form genetically identical clones. At many higher plants clones can be received by asexual reproduction (tubers, bulbs, shanks, etc.). E.g., all plants of one grade of potatoes are genetically identical and therefore distinctions between them are caused by external factors. If to grow up two different grades of potatoes in identical conditions, then distinctions between plants will be connected with distinctions in their genotypes. At animals clones of genetically identical individuals can be received as a result of close and long inbreeding (see). For identification of a role of heredity and Wednesday in formation of various signs at the person with success use twin method (see). Researches on the monozygotic twins having identical heredity and who are brought up separately supply with the valuable information on influence of the environment on development of ancestral features. In the analysis discontinuous (qualitative) And., caused by alleles of one gene locus to define the nature of inheritance and a contribution to the general And. hereditary and environmental factors usually does not represent big work. In case of continuous (quantitative) And. the researcher meets considerable difficulties. During the studying of any quantitative character carry out the analysis of perhaps bigger number of the individuals differing among themselves and the obtained data group in a row dimensional classes. Having presented these sizes in the form of the schedule of distribution of frequencies, it is possible to investigate I. Obychno's character it is difficult to divide genetic and environmental components of variability and to investigate separately that its part, edges it is caused genetically. Various mathematico-statistical methods are developed for studying of these components (see. Biometrics ).

Value

Variability and heredity — two parties of the same circle of the phenomena. The descended changes represent material for selection. And., heredity and selection of organisms, fittest to conditions of this environment, make the main motive powers of evolutionary process. Ch. Darvin and A. R. Wallace's huge merit is discovery of evolutionary value of selection and development of the theory of this question (see. Theory of evolution ).

Fig. 3. Typical (below) and melanichesky (at the top of) a form of butterflies — Biston betularia.

Selection hereditarily of the changed forms is a major factor of speciation. For the last 130 years in some areas of Northern Europe and North America in connection with environmental pollution dark forms of butterflies appeared, and in many of these areas such forms began to prevail. Such discolorations of butterflies are noted by hl. obr. in districts with strongly developed industry, in particular in England. In 1848 the dark form of a butterfly of Biston betularia made about 1% of population, and in 1894 in the same district its relative number already exceeded 99%. Such phenomenon of «an industrial melanizm» is connected, obviously, with pollution of the woods a soot in industrial districts: typiforms of butterflies (light, with small specks) are more sharply allocated on the trees covered with a soot in comparison with melanichesky forms and more often eaten by birds. Therefore, in the contaminated districts melanichesky coloring is protective (fig. 3).

The high number of populations of insects — wreckers of page - x. plants provides a sufficient reserve And. in order that these insects could have lines steady against the poisons used against them. At the same time continuous processing by insecticides of extensive territories creates a powerful selective factor of the few steady mutants. So, DDT and other insecticides in wide scales began to apply in 1944, and by 1960 more than 120 species of insects, including apprx. 60 types having medical value which had resistance to DDT or other insecticides were revealed. Many examples fiziol, the stability connected with development of the various mechanisms interfering penetration into an organism of an insect of an insecticide or its action are described. Emergence of a number of strains of the microorganisms high-steady against antibiotics is known (see. Medicinal stability of microorganisms ). It, in particular, is connected with the fact that doctors quite often abuse antibiotics, registering them in underdosages when other means could be more effective.

Definition of a sign many genes, i.e. polygenic system, causes high stability of a phenotype. E.g., the single mutation will hardly seriously break manifestation of the sign depending on the additive effects of genes of 30 and more loci. Selection favored to development of similar systems since they reduce harmful effects of such insignificant events as replacement of one couple of bases in DNA. On the other hand, the polygenic systems which are characterized rather small And., have huge potential ability to And. as a result of various combinations of genes. Under the influence of selection there can be an increase in number of organisms with such genotypes which in the absence of selection would never become mass.

Polygenic systems are the important mechanism of maintenance of balance between fitness to conditions of this time and flexible adaptability to slow, long changings of the environment.

Main categories I. — hereditary and modification — are always present at a harmonious combination and are important for existence of live organisms. If on the basis of the first of them selection creates an adaptive reaction norm, then by means of the second category I. the hereditary reaction norm is implemented in specific conditions of the environment, providing reasonable response of the individual to these or those influences of external factors. Opponents of genetics imposed to biology the lamarkistsky doctrine in due time, according to Krom the main thing in development of organisms is the environment. This concept substituted the internal reasons of development by external influences of the environment.

Fig. 4. Decolourization of the Himalaya rabbit at various heating environments: and — the rabbit who is grown up at a temperature over 30 °; — the rabbit who is grown up in normal conditions at a temperature of 25 °; in — after the placement of a white Himalaya rabbit to the cold room on site of the pulled-out white hair black grow.

Insolvency of the doctrine «about a crucial role of external environment» in development of organisms is obvious now. However it would be wrong to consider that Wednesday does not play any role in development of an organism. Environmental factors quite often exert noticeable impact on manifestation of the signs controlled by genes that leads to their strengthening or easing. During the studying of influence of external factors for the purpose of management of development of organisms it is necessary to reveal first of all those conditions in which hereditary opportunities fullestly are implemented. So, typical coloring of the Himalaya rabbit — white with a black nose, ears, a tail and tips of pads — develops at cultivation of an animal indoors with a temperature about 25 ° (fig. 4, b), and at a temperature over 30 ° the rabbit grows completely white (fig. 4, a). If to pull out white hair and to place a rabbit in the cold room, then again grown hair will be black color (fig. 4, c). The chromogenesis at a Siamese cat is controlled by a gene with similar temperature dependence, i.e. this gene provides products of a pigment at the lowered temperature. Therefore, as well as at the Himalaya rabbit, at this animal only the most acting parts of a body are painted. The Siamese kittens growing on cold are more dark than the kittens developing in the warm room. If to transfer a certain kind of Primula sinensis with red flowers to a greenhouse and to grow up at high temperature and big humidity, then through certain time again blossoming flowers will be white; if to lower temperature to 15 — 20 °, then on the same plant red flowers appear again.

Says the fact that seeds from white and red flowers equally develop in the plants forming red flowers at low and white flowers at high temperature about lack of hereditary changes in the course of this experience. Therefore, the genes controlling a chromogenesis do not show the effects at high temperature. Cultural highly productive forms of animals and plants are especially sensitive to influence of environmental factors. I. V. Michurin in the selection work attached it great value. Gena controlling so-called quantitative characters (e.g., growth, weight, heart rate, size of arterial and venous pressure, etc.), «are very sympathetic» on environmental factors. As an example of interaction of a genotype and exogenous factors emphysema of lungs can serve in an etiopathogenesis of diseases of the person, edges it is initially caused by hereditary insufficiency of alpha antitrypsin. At homozygous people emphysematous changes in lungs develop, and they, as a rule, perish in the middle age. Earlier believed that the heterozygous individuals revealed biochemical are not ill. However later it was established that at heterozygous people emphysema nevertheless arises when their lungs are exposed to harmful effects (nicotine, industrial wastes in air, etc.).

For definition of an etiology of these or those diseases and inborn malformations first of all it is necessary to find out from the person whether these aberrations are hereditary or they are caused by teratogens, i.e. are not hereditary. In case of the hereditary nature of this or that defect it is necessary to define correlative value of genetic factors and Wednesday in manifestation of this defect since for development of methods of therapy and prevention of hereditary anomalies it is important to know not only their pathogeny, but also possibilities of change of manifestation of a mutant gene. For studying of mechanisms of development and for development of methods of treatment of hereditary diseases mutant lines of the animals having similar anomalies with the person can be used with success. Many mutant genes are transferred to closed lines of animals that considerably facilitates clarification of a role of heredity and the environment in emergence of this or that anomaly. Manifestation of effects of mutant genes at the higher organisms in many cases gives in to change; it opens great opportunities for development of effective methods of correction of hereditary anomalies at the phenotypical level, i.e. normalization of development of an organism without change of a genotype. On the other hand, considering that manifestation of effects of genes depends to a large extent on influence of environmental factors, it is necessary to protect the developing germ of the person from the damaging effect of teratogens that can reduce the frequency of not hereditary inborn uglinesses considerably.

Variability of bacteria — see. Bacteria (genetics).

See also Inheritance , Hereditary diseases .



Bibliography: N. P. Genetikachelovek's tanks, M., 1978; Vavilov N. I. Chosen compositions, Genetics and selection, M., 1966; Dubinin N. P. General genetics, M., 1976, bibliogr.; B. V. grooms. Biological modeling of hereditary diseases of the person, M., 1969, bibliogr.

B. V. Konyukhov.

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