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GENETICS AND ITS ROLE IN BIOLOGY

by

Y.S. Demin
Institute of Developmental Biology
Academy of Sciences
Moscow, U.S.S.R.

1 INHERITANCE AND VARIABILITY

Genetics is the science of inheritance and variability. Inheritance is the capacity of the offspring to acquire the characters and peculiarities of development of the parents. Variability is contrary to inheritance, being the ability to change heriditary factors, as well as the manifestation of their properties in the process of development.

The most remarkable feature of the reproductive process is that the young show significant similarity to their progenitors, all species of plants and animals producing offspring resembling themselves. Individual features are inherited by each generation with great precision.

No matter how great the resemblance between relatives, it hardly ever appears to be complete. Members of the same family will always differ from each other; and most or even all of them will differ from their parents, not to mention more distant relatives. In some cases the variability related to distribution of different characters among offsprings operates according to definite laws of inheritance, while in other cases it is related to environmental conditions. Variability due to new gene combination is called combined hereditary variability. For example, when crossing two plants having red flowers, one can get white variants in the offspring. In the process of development and vital activity of the organism influenced by both external and internal conditions, hereditary changes appear due to changes of genetic structure and this is mutational variability. When variability is due to a modified manifestation of a character, due to environmental conditions, it is called modification. Here it is not the definite manifestation of the character having a modified variability that is inherited, but the whole range of variation of the character depending on the environment - the so-called reaction norm of the organism.

Strictly speaking, there are no hereditary changes, but all the character changes are hereditarily determined. Inheritance is the phenomenon which not only ensures resemblance, but also variations, of organisms in a number of generations. These variations appear as a result of variability of hereditary characters. Therefore, inheritance and variability are the two parts of the same process which makes for the evolution of organic forms.

Thus we differentiate between hereditary variability conditioned by a genotype and maintained in a number of generations, and non-hereditary variability which is modified changes in the phenotype of the organism.

2 THEORY OF INHERITANCE OF ACQUIRED CHARACTERS

The theory of inheritance of acquired characters was put forward as early as 1809 by J.B. Lamarck. He was the first to point out the paramount role of the environment in hereditary variability, and it was not until the nineteenth century that the theory of inheritance of acquired characters was subjected to critisism by a number of specialists. The most outstanding critic was A. Weismann who elaborated his ideas during the period 1883–1892. He pointed out the differences in the plasma of generative and somatic cells. The generative (germ) cells are the only ones that participate in sexual reproduction determining continuous transfer of hereditary characters from generation to generation. Hereditary characters of an organism are determined by germ plasma, while the hereditary changes are determined by the changes in its molecular structure; hence, Weismann theorized on the non-inheritance of characters acquired by the organism during its life cycle. Further development of experimental investigations on population genetics proved the conception of inheritance of acquired characters to be wrong.

In fact, following the logical sequence of the statements, we might admit that the character change under the influence of environment leads to adequate change of hereditary structures (genes) regulating this character. Such a change should have brought about the hereditary fixation of a given character. Though mutations (hereditary changes) result from the influence of environment, they do not appear to be adequate to bring about hereditary fixations. Mutations occur in many directions; they can be adaptive for the organism since they take place in separate cells, their action being manifest in the offspring. The adaptive role of mutation is determined by selection and not by its causative factor. The direction and rate of evolution are determined by natural selection, the latter being regulated by any factors of the outer and internal genetic environment.

The question of inheritance of acquired characters has been analyzed in detail because this theory is still upheld by a number of biologists, selectionists in particular, due largely to inadequate knowledge of the basic factors regulating the relationship between genotype and environment.

3 MATERIAL BASES OF INHERITANCE

The study of inheritance and variability is sure to stimulate the investigation of material bearers of the organism's properties. Thus, apart from inheritance and variability genetics is the study of the material bases of inheritance, i.e., cell structures and functions which determine the nature of an individual's development. All the elements of the cell possessing the ability for self-reproduction and segregation into daughter cells in the process of division constitute the material basis of inheritance. Conventionally we differentiate two types of inheritance, nuclear and cytoplasmic. Nuclear bearers of hereditary information are chromosomes. They play a significant part in determining genetic constitution of the cell and the organism as a whole. The bearers of non-nuclear (cytoplasmic) inheritance are the cell elements capable of self-replication: mitochondria, chloroplasts, centrioli, etc. The information transported by cytoplasmic structures is primarily determined by chromosomes genes.

Thus, studies in the field of genetics should deal with both inheritance and variability, two opposing and closely inter-connected processes, as well as the material bearers of hereditary information. The investigations should cover all structural levels of living organisms: molecular, chromosomal and cellular, as well as the organism as a whole and its populations.

4 METHODS OF GENETIC ANALYSIS

Genetic analysis is generally based on the study of characters developing in a number of generations.

The principal means of genetic analysis is hybridologic analysis1 which is applicable at all structural levels of the living matter, from molecule to body. This method is used in the investigation of hereditary structure, behaviour in various systems, interaction of genes and incorporation of characters by genotypes. Hybridologic analysis makes it possible to establish all the principal concepts of genetics.

Cytological methods help to investigate material bases of inheritance, namely, anatomy of hereditary structures.

Cytogenetic methods came into being as a result of the combination of cytological and genetic methods. As such, it analyses the behaviour of hereditary structures in cell generations. Modern genetic analysis includes mathematical, biochemical, biophysical and other methods of investigation.

The possibility of application of exact qualitative and quantitative methods of analysis places genetics alongside physics and chemistry within the range of exact natural sciences.

1 Hybridologic analysis is sometimes identified with recombination analysis. It does not seem to be correct since recombination analysis is only a part of hybridologic analysis.

5 HISTORY OF EARLY DEVELOPMENT OF GENETICS

Regularities of inheritance were first detected in plants, and that was not incidental. Plants are easier to study than animals for inheritance regularities. It was only during the current century that it became possible to study and pinpoint the characteristics in animals that were much more suitable for investigation than in many plants. That was achieved, however, on the basis of application of the discoveries made on plants. Geneticsas a science came into being as a result of studies on animal and plant breeding.

By selecting and crossing the best representatives of any group from generation to generation, relative groups, i.e., strains, breeds and brands having their own characteristic hereditary traits, were created. Originally observations and comparisons were too few to be formulated into an independent science. In the second half of the 19th century, however, the rapid development of cattle breeding and plant breeding (seed-farming in particular) resulted in increased interest in problems of inheritance.

The first systematic work on inheritance and variability problems was Darwin's “Origin of species”. The author managed to collect a large amount of data, though Darwin failed to establish regularities of inheritance at that time.

The progress in experimental biology at the end of 19th century was of great importance in establishing genetics as a science. In the field of cytology basic regularities in processes of mitosis and meiosis were discovered, as well as the permanent number of chromosomes in cell nuclei of each species. T. Boveri, E. Van-Beneden, et al., proved that gamete fusion at fertilization restored the original number of chromosomes which are constant for each species. They proved that in a zygote, the first cell of the future organism, half the number of each parent's chromosomes is incorporated.

At the end of the 19th century, a few theories were formulated which attempted to explain the phenomenon of inheritance. All of them are only of historical interest. We should dwell in some detail on one of these theories, namely, Weisman's theory of “germ plasma and its continuity”. Weismann put forward an explanation for reduction division, the process of which was not clear then. Maintaining histheory of natural selection in 1886, Weismann postulated the role of sexual reproduction in hereditary variability. He suggested that the latter was conditioned by a mixture of different germ plasmas which occurs at sexual reproduction. It is this very process that provides for the constant occurrence of individual hereditary changes in offspring, which is the material for natural selection. Weismann suggested an explanation of the essence of two reduction divisions preceding each conception. The process of reducing the number of germs inherited from ancestors is considered by Weismann to be the reason for constant change of germ unit sets that get into the organism in any generation. Thus it is these processes of reduction division with the subsequent fusion of each parent nucleus occurring at every sexual reproduction that is responsible for endless diversity of inherited traits in offspring. 1 Hereditary germs, or ids, in aggregates from chromosomes. At crossing there takes place an everlasting diversity of ids; hence Weismann's scheme of hereditary cell combinations.

At present Weismann's theory is only of historical interest. However, this explanation of reduction division and recombinations of hereditary germs is very close to our modern conception of the processes of generative cell formation. Besides, Weismann's hypotheses on the essence of reduction division and of germ plasma made biologists deal with combinations and probability in respect to heredity phenomena.

In his “Experiments with plant hybrids”, which was published in 1865 Mendel discussed the main regularities of inheritance and pointed out in particular that:

  1. Characters are determined by certain hereditary factors that are transferred through sex cells; and

  2. Some characters of the organisms do not disappear but are preserved in the offspring in the same form as in the parents, though they may not always become manifest.

Mendel's theories are of great importance in understanding evolution. They reveal one of the sources of variability, namely, the mechanism of preservation of adaptive characters of species in a number of generations. If these adaptive characters were suppressed or disappeared at crossing, the development of the species would be impossible.

Mendel's regularities were reconfirmed by H. de Vries (1848–1935), C. Correns (1864–1933) and E. Tschermak (1871–1962).

De Vries crossed mainly very close species distinctly differing in such obvious characters as flower colouration. He observed the phenomena of dominance and segregation in maize, poppy and many others. He published his observations in 1900, mentioning that the most significant points of these conceptions had been stated much earlier by Mendel in his work on the pea. They had been forgotten, however, and remained unrecognized. De Vries was the first to introduce such notions as monohybrids, dihybrids and polyhybrids.

Carl Correns (1864–1933) started his investigations of hybrids in 1894 and made a series of experiments crossing maize, peas, lilies and gilly flowers. In his article, published in 1900, on the results of hybridization experiments, Corrence first described the main principles of Mendel's discovery, whose paper he had become acquainted with in 1899. Then he presented his own data based only on crossing peas. It is of interest that Corrence visualized the possible existence of a cytological splitting mechanism. He mentions the sexual cell nucleus as the bearer of dominant and recessive dispositions and adds that the numerical ratio of gametes 1:1 suggests the idea of nuclear division, in other words, Weismann's reduction division.

In 1898 Eric Tschermak started experiments on hybridization of peas. In 1899 he discovered some regularities in peas, and shortly afterwards he came across Mendel's paper. In 1900 Tschermak presented his own paper for publication.

The discovery of hereditary regularities was immediately picked up by many scientists who devoted themselves to a new series of experiments that confirmed and developed those made by Mendel. It is evident that Mendel's discovery and the confirmation of his ideas was a natural result of research in the field of reproduction, fertilization and evolution, as well as variability and inheritance.

1 Later on Weismann (1892) attributed a greater role to the influence of the environment, in the appearance of new hereditary changes.

6 MENDELISM AND THE THEORY OF EVOLUTION

Mendelism played a decisive role in the elucidation of important aspects of Darwin's teaching and served as the basis of modern teaching on natural selection. But it was not realized that Mendel's ideas did not find due scientific recognition, and his first followers were attacked severely. The controversy went so far as to oppose Mendelism with Darwinism. Though inheritance as the basis of Mendelism does not entirely coincide with the theory of evolution which is the basis of the teaching of Darwin, they are nevertheless closely interrelated.

In his speech “Charles Darwin and half-century results of Darwinism” (1909), K.A. Timiryazev pointed out that Mendelism undermined Jenkin's objection (to Darwin's theory) that crossings ought to bring out levelling and bogging of hereditary deviations, bringing the role of selection to naught. The most important result in this respect, said Timiryazev, is the fact that characters do not fuse, do not mix or divide, do not tend to disappear but remain unchanged, and are distributed among different offspring.

The controversy concerning Mendelian regularities stimulated scientific thought and resulted in the emergence of ideas and theories that, despite the controversy, played a certain role in the development of evolutionary theory and genetics. Most important of these was the theory of mutation of H. De Vries (1901–1903) which, according to the author's original ideas, completely solved the problem of species formation. The following concepts of H. De Vries' theory are still valid;

  1. Mutations occur suddenly, without any transitions.1

  2. New forms are quite constant and stable.

  3. Mutations, in contrast to non-inherited changes (fluctuations), do not form continuous ranges, do not group round the mode. Mutations are qualitative changes.

  4. Mutations occur in different directions; they may be either useful or harmful.

  5. Mutations can occur repeatedly.

De Vries held the theory of mutation against that of natural selection. He believed that mutation can give rise to new species; in this case no natural selection would be required. In fact, mutations are only a source of inherited changes, the latter being the material for selection. At present we know that gene mutations are estimated by selection only in a genotype system. In spite of De Vries' faulty concepts in his mutational theory, the fact that he drew the attention of researchers to mutation as an object for investigation has done him credit. He predicted the significance of mutations for selection “....mutation, mutating itself should become the object of investigation. If we ever manage to find out mutation laws, then our concept regarding mutual relationship of living organisms will not only become much more profound but we shall also have opportunity to regulate mutability as much as a selectionist governs variability. There is no doubt that we shall come to it gradually managing certain types of mutations which will do a lot of good to farming and gardening....”

It seems that there is considerable scope for significant work on mutation. The study has become the principal mechanism of gene apprehension; and now we are on the verge of getting to know the mechanisms of mutations.

1 De Vries considered mutation to be uneven change of a hereditary character.

7 GENETIC STUDY OF POPULATIONS

The formulation of Mendel's regularities was the basis of genetic study of populations. The study of the effect of genetic laws in populations has brought about the science of population genetics.

The first investigation of population genetics, combining genetic analysis and mathematical analysis, was carried out by W. Johannsen. In 1903 he published his paper “On inheritance and populations of pure strains”. Theoretical conclusions of his results were the first to point out the essence of the processes going on in natural and artificial selection.

Self-fertilized plants, such as barley, French beans and peas, were chosen by Johannsen as an object for his investigation of population. As far as the method was concerned, the work was much simplified, since each population could be divided into groups of offspring of certain plants; i.e., to isolate certain pure strains1. The weight and size of seeds served as characters. According to the latter, there is a marked modificational variability. There used to be different viewpoints as to the role of variability in evolution. Those supporting the theory of inheritance of acquired characters considered modifications caused by the influence of environmental factors to be inheritable. The opponents of this theory denied the inheritance of modificational changes. The settlement of the argument in favour of the latter was of great importance, as the selection of the organisms according to phenotype without detecting hereditary potentials was very common in selection and retarded the breeding of new animal and plant species. Yohannsen showed that selection in populations causes a shift big or small - in respect to selection - of that intermediate character around which corresponding individuals vary, and that within pure strains regression was complete; the selection within a pure strain did not result in any shift of this type. These conclusions were of major importance. They confirmed the principle of individual test of offspring, which was applied in selection by the French selectionist, Le Vilmorim (1816–1860). According to it it was not enough to choose species with best indices in selection. It was also necessary to check according to offsprings investigated individually for each parent form as to whether the chosen character would be typical for them. This principle holds good even today.

Johannsen found the selection to be effective only in populations with hereditary characters present in them in a desired direction. As soon as these characters become manifest in the population, selection ceases to be effective. Therefore selection within pure strains is not effective at all. Hence the conclusion of non-inheritance of acquired characters and inadequacy of selection according to phenotype alone

Johannsen formulated a number of theoretical concepts of genetics. He suggested such terms as gene, genotype and phenotype. The term gene was proposed by Johannsen to be used for a hereditary factor found in a sex cell and inherited independently, and from this the term genotype which is the aggregation of all hereditary dispositions determining the organism's development. Genotype can be opposed to phenotype. Species phenotype is the whole combination of its individual characters available for observation and analysis. Phenotype is not merely a combination of simple characters, it is the expression of very complicated interrelations. (Johannsen, 1926).

Johannsen's work on pure strains caused a revolution in the knowledge of selection processes, and stimulated further research with various subjects, ranging from micro-organisms to higher forms. It was only in self-fertilized plants that investigators dealt with pure strains. Asexual reproduction or parthenogesis offspring originate from only one individual; therefore the genotype of the original parent form is reproduced in the offspring. Such an offspring is called a clone. If sexual reproduction occurs with close inbreeding, we speak about ever increasing homozygosis of consanguineous lines. It is not necessary to dwell on the investigations aimed to check Johannsen's concepts. It should suffice to point out that they were fully confirmed. The observations of Johannsen remain valid and indisputable for scientists working in the field of genetics and selection.

Another pioneer in the field of genetics was W. Bateson, the author of the first papers on complex Mendelian analysis. His studies disproved any correlation between the number of characters and the number of hereditary factors, as well as of their identical independence. Bateson was the first to point out incomplete domination, complex allelomorphism and other deviations from the typical Mendel's concepts. He was also the first to explain cases in which the character of an animal or plant depended not only on one pair of factors but on two or more, which brought about change in the numerical ratio in the offspring.

Bateson suggested the terms homozygotes and heterozygotes, which have retained their original meaning up to date. Later on, his theoretical work (theory of presence-absence) took him away from the main path of genetics development. However, he made a great contribution to the development and popularization of Mendel's ideas.

1 Johannsen called pure strains those individuals derived from one self-fertilized individual. Hence, the population of absolute self-fertilizers consists of only pure strains, individuals of which may coexist in nature but cannot be involved in crossing.

8 DEVELOPMENT OF MODERN GENETICS

The foregoing were the theories and experimental data that formed the foundations of genetics in the first stages of its development. They nearly coincided with the second discovery of Mendel's laws, lagging behind them only a little, and confirming once again the adequacy of biology for the understanding of the new science of genetics. As soon as genetics became established as a science, biology made a stride, ceasing to be a descriptive science and joining the ranks of exact natural sciences.

The second stage in the development of genetics started in the second decade of our century. The most important development of the period was the establishment of the American genetic school of chromosome theory of inheritance headed by T.H. Morgan, showing clearly that hereditary factors were related to chromosomes, and elucidating the role of chromosomes in sex determination as well as sex segregation (as 1:1). The chromosome theory of inheritance explained the exceptions to Mendel's laws and their universality for all organisms reproducing sexually.

It is at this stage of development that genetics found its application in selection. In 1922 N.I. Vavilov formulated the law of homologous rows in hereditary variability. The essence of the law was that plant and animal species of related origin have similar rows of hereditary variability. Vavilov stated that:

  1. Genetically close species and kinds are characterized by similar rows of hereditary variability with such regularity that knowing a number of forms within one species one can foresee the availability of parallel forms in other species and kinds. The closer the species and kinds are arranged in the general system, the greater is the resemblance in the rows of their variability.

  2. Whole families of plants are generally characterized by a definite cycle of variability; hence regularities of close species and kinds in polymorphism make it possible to predict the availability in nature or in artificial selection, of respective forms by means of mutation, in-breeding or hybridization.

Applying this law, Vavilov defined the centres of origin of cultured plants to be where the largest variety of hereditary forms is collected. Bearing in mind hereditary variability of any species, one could fill in the missing links of the species relative to the former on the basis of the law of homologous rows. The law of homologous rows is of great importance to breeding new kinds of plants and animal species.

Another event, the significance of which was realized to the full extent only much later 1 was the discovery of mutagenic effects of X-rays on fungi (in 1925 by G.A. Nadson and G.S. Filipov and on Drosophila in 1927 by H.J.Muller). To get an idea of the revolution that this discovery made in genetics, it is worthwhile recalling that genetic analysis is based on the study of mutations; i.e., different forms of existence of one and the same gene. With the discovery of the mutagenic effect of radiation, there appeared a possibility of increasing mutation yield by hundreds and thousands of times. While this discovery limited the use of theoretical and experimental genetic analysis, to the applied genetics especially the selectionists, it gave a tool of considerable importance; i.e., a number of mutations in the time unit, which made it possible to intensify selection work. These reports were the first to show that gene variability was affected by environmental factors. The study of the influence of ionizing radiation on inheritance resulted in the emergence of a most important branch, the radiation genetics.

1 Since the forties in connection with radiation hazard.

During the same period S.S. Chetverikov (U.S.S.R.) started elaborating theoretical bases of population genetics. In the thirties, V.V. Sakharov and M.S. Lobashev in the U.S.S.R. and later S. Auerbach in England found hereditary changes influenced by chemical substances. In other words, chemical mutagenesis was discovered. It is of interest that the real progress in investigation on chemical mutagenesis could be witnessed only in the fifties, after the main concepts of gene structure had become available.

In the thirties a group of Soviet geneticists established some facts concerning the structure and function of genes. These findings anticipated the discovery of molecular genetics made a few years later and appeared to be the starting point of molecular genetics of higher forms. The discovery of pseudoalleles by a group of geneticists led by A.S. Serebrovsky showed that the gene is a complex unit consisting of several parts. Data obtained showed the possibility of crossing-over within one and the same gene; i.e., the possibility of gene-division.

Another worker, B.N. Sidorov showed changes in dominating characters in respect to gene arrangement in the chromosome (it was convincing evidence of position effect 1, discovered by A.H. Sturtevant in 1925. But more important was the establishment of another fact, namely, feed-back dislocation effect. If the arrangement of genes in a chromosome returned to the original state, there would be no observed changes of gene effect. These facts indicated the significance of space arrangement of genes providing for their activity.

The period since the forties to the present time can probably be called the period of molecular genetics. This period is characterized by studies employing a combination of genetic and biochemical analyses, as well as physico-chemical and mathematical methods. Owing to this as much as to the use of micro-organisms, findings were made on the chemical structure and activity of the hereditary factors, the genes. Deoxyribonucleic acid (DNA) appeared to be the messenger of hereditary information of all cellular forms, and the manifestation of gene activity is concerned with the synthesis of specific protein-enzymes.

1 Position effect is such a change of gene location in a chromosome which resulted in the change of this gene manifestation.

9 GENETICS AS AN APPLIED SCIENCE

In the role of genetics as an applied science we have to differentiate two aspects. On the one hand are theoretical problems and their solution. Of paramount importance are such problems as structure and reproduction of hereditary material, the problem of mutations (gene changes), interrelation of a gene and a character, interrelation of inheritance processes, variability and selection in evolution and the problem of gene interaction. It is difficult to predict at the present stage of development, the applied aspect of these, but the solution of the problems is necessary for the progress of genetics as science. On the other hand, there are practical problems that call for solution, such as ways and means of increasing productivity in livestock-raising and plant-breeding, the problem of hereditary diseases, protection from harmful physical and chemical hazards (e.g., radiation) and so on.

At present there are areas where hybrid maize is more stable and abundant than pure strains. Hybrid seeds are obtained by crossing plants of different strains. Since maize is a monoecious plant, the plants of the maternal line should be deprived of male inflorescences (tassels) so that female inflorescences (pistils) are pollinated by pollen of another line. Other hybrid seed production is being conducted on a large scale; cutting male inflorescences requires much effort.

On the basis of inheritance studies of maize, geneticists have discovered the phenomenon of cytoplasmic male sterility, where the pollen is unable to pollinate female inflorescenses. By sowing this kind of maize as the maternal strain next to plants having regular pollen, it is possible to secure pollination of pistils by the proper pollen without the time-consuming work of cutting male inflorescences.

At present the phenomenon of cytoplasmic male sterility is also used to obtain hybrid seeds of sorghum, onions, beet and other plants.

After a long period of beet selection using routine methods it was found that the possibility of further increase of its saccharine level was limited. However, the application of more exact genetic methods (the increase of the number of chromosome sets ploidy) made it possible to grow triploid sugar beets (Bete vulgaris var.saccharifera), the saccharine level of which was higher by 1–1.5 percent, while the yield of beet tops increased by 20–30 percent. It is very important for selection purposes to control the change in chromosome number (ploidy) in the cell. This is the means to increase to a great extent the yield, resistance to diseases, etc.

The investigation of the problem of hereditary variability by inducing mutations under the influence of chemical substances, ionizing radiation, and ultra-violet light is also important for the elaboration of new methods of selection.

In recent years, genetics has acquired even greater importance in solving medical problems. According to available estimates, about 0.03 percent of each generation of human populations be affected by various hereditary illnesses. These include such severe ailments as schizophrenia, cretinism, haemophilia, etc. The progress of medical cytogenetics within the last decade shows that several hereditary illnesses are due to structural disturbances of the chromosomes. It may now be possible to work out new methods for prevention and control of these diseases.

The more extensive use of atomic energy raised the problem of radiation hazards. It is known that radiation affects both body cells (somatic cells) and sexual cells (generative cells). Radiation results in changes in genes and chromosomes. There occurs some kind of disturbance in somatic cells that brings about radiation disease and in some cases neoplasms, but they are limited to the same generation; whereas any changes in generative cells lead to mutations that may be inherited by the descendant generations. It has been established that under the influence of radiation there occurs a large number of lethal mutations, their frequency being dependent on the radiation dose. The effect of the doses must be summed up for proper estimation. It has been shown that the effect of regular small radiation doses results in accumulation of harmful (negative) mutations that can progressively increase hereditary lethality, malformations and other severe diseases, the hereditary abnormalities which will become manifest in future generations. Therefore it is evident that radiation may present a great genetic hazard unless appropriate protection measures are taken. (Unfortunately we have not yet learnt how to treat mutations).

Genetics has greatly helped the pharmaceutical industry by artificially inducing hereditary changes in the micro-organisms which produce antibiotics. These mutations were induced by ultra-violet light, chemical reagents and X-rays. It was possible in this way to increase the yield of certain strains from 200 up to 5 000 units, thus decreasing the production cost of antibiotics by 25 times.

The etiology of malignant tumors of somatic cells (cancerous) is still enigmatic, this being the reason for lack of effective ways to control the malady. Large number of specialists in the field consider the changes of hereditary apparatus of somatic cells to be responsible for the origin of cancer. This kind of change may result from mutation. The theory has been confirmed by a number of experimental findings, and one may hope that genetics will contribute substantially to the cause of cancer control.

10 GENETICS IN RELATION TO OTHER BIOLOGICAL SCIENCES

The main task of biology is the study of living matter, and in this respect the role of genetics is especially important since it covers two basic phenomena - inheritance and variability. These are related to reproduction, which in its turn has a physiological and biochemical basis. Individual development is determined primarily by manifestation of gene action in the process of ontogenesis, which means that genetics has a direct relation to embryology, besides physiology and biochemistry.

In the study of taxonomy, genetic methods are often used. In fact, specificity of chromosome pattern has long become one of the systematic tests. At present, methods of molecular genetics are being worked out, for instance, complementary hybridization of nucleic acids in application to the problems of systematics and evolution. It is hoped that in the near future these methods will provide means for establishing new evolutionary relationships between organisms.

The application of genetic methods has made it possible to solve a number of problems specific to such sciences as biochemistry, physiology and embryology in a new and a more efficient way. By making use of hereditary changes and mutations, one can switch off and on almost all physiological processes, interrupt biosynthesis of metabolites in the cell, interrupt morphogenesis and the like. The use of genetic methods in these sciences enables us to obtain nearly any model necessary for investigation of one or another problem.

Genetics plays quite a special role in the teaching of evolution. Inheritance variability and selection - are the main factors of evolution. The role genetics played in consolidating Darwin's ideas has already been elaborated. Selection involves the effect of gene action, and it is owing to the evaluation of gene action expressed in characters and properties of the organism that selection of genes occurs, creating a most valuable system, a genotype. That is how natural selection operates to create a definite genotype of the organism. Thus genetics has unravelled the main factors and mechanisms of interrelationship in evolution - inheritance, variability and selection.

Thus we can see that genetics is connected with all theoretical and applied biological sciences, including medicine and agriculture. Inheritance and variability is inherent in all nature, and this determines the position of genetics in the whole system of biological sciences.

11 CONCLUSIONS

Genetics is the science of inheritance and variability of those properties that are inherent in all living matter. While dwelling on the main trends in theories and hypotheses that have laid the foundation of modern genetics (Mendel, De Vries, Weismann, Bateson and others), only those concepts have been mentioned that have retained their significance up to the present or played a positive role earlier. No criticism of faulty concepts of these theories have been made because it was not these concepts that determined the progress of the science. Even an outstanding scientist may sometimes propose faulty concepts. It may be worthwhile mentioning again that the development of genetics is the logical sequence of Mendel's concepts. Findings and discoveries of recent years did not require any revision of regularities of Mendelism in the least, but have yielded new proof of their reliability.

12 REFERENCES

Auerback, C., 1963 The science of genetics. New York, London

Correns, C., 1902 Über Bastardirungsversuche mit Mirabilis-Sippen. Ber.Duetsch.Bot.Ges., 20: 540–608

de Vries, H., 1901 Die Mutationstheorie. Veit, Leipzig.

Mendel, G., 1866 Versuche über Pflanzen-Hybriden. Verh.naturf.Vereins, Brunn, 4

Norgan, T.H., 1919 The physical basis of heredity. J.B. Lippincott Co.

Sturtevant, A.H., 1925 The effect of unequal crossing over at the Bar locus of Drosophila. Genetics, 10:117–47

Vavilov, N.I., 1966 The principle of homologous lines in hereditary variability. In book “N.I. Vavilov, Izbr.sochineniya” M., publ. “Kolos”.


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