GENETICS. - Genetics is defined as the science of heredity, that is, the doctrine of the phenomena concerning the reappearance of the same character in successive generations of a living organism, not directly induced by external causes. Therefore genetics does not include the study of characters induced by the persistence of environmental factors (nutrition, light, humidity, parasitism, etc.), which likewise repeat themselves in the succession of generations. This fundamental distinction acquires a special importance in the study of pathological characters, which are considered hereditary only when the cause of the character cannot be imputed to the external factors mentioned: for example, syphilis is not considered hereditary but congenital, because it is due to contagion transmitted during fetal life.
Genetics has as its first objective the knowledge of the material substrate determining hereditary characters, which are thus conceived as the result of an action proceeding from within the organism itself. This material substrate takes the generic name of hereditary patrimony or genotype: it is demonstrated that it repeats itself, more or less unchanged, in all the cells of the organism, and resides chiefly in the nucleus. The nuclear hereditary patrimony constitutes the genome, in turn composed of a great number of corpuscular units, called genes (v.), aligned in the chromosomes.
Genetics took shape around the laws of the heredity of crosses, discovered by G. Mendel in 1865. They can be synthetically set forth as follows.
I law: if two individuals differing in one character are taken (for example, peas with green seed-peas with yellow seed) coming from strains constituted of identical individuals (strain with green seeds-strain with yellow seeds), a generation is obtained all alike with only one of the two characters (in our case yellow seed).
II law: if two individuals with yellow seed are crossed with one another, but offspring of yellow seed and green seed (that is, two siblings of the generation referred to in the I law), a mixed progeny is obtained with 1/4 green seeds and 3/4 yellow seeds. The parents of this generation are called hybrids or heterozygotes, while those of the preceding generation (I law) are called pure or homozygotes.
III law: if individuals differing in several characters are crossed (for example, two characters of the seed, yellow and green; two of the stem, tall and short, etc.), each is inherited independently of the other according to the first two laws. The three laws of Mendel remained ignored until 1900, when De Vries, Tschermak and Correns rediscovered them, reconfirming them on various botanical material. Since then the laws of Mendel have become the foundation on which genetics has been built, so that one cannot distinguish the study of hereditary phenomena called Mendelian, that is, which follow the laws of Mendel, from the development of genetics itself. Only certain completions were brought to them. Thus today it is documented that dominance and recessiveness (I law) are often replaced by intermediate heredity (pink flowers, from red flowers × white flowers). Moreover the III law today has a rather limited value, though it has not lost its importance. When it holds, it means that the characters considered are due to genes located in different chromosomes (v. CROMOSOMA). When the III law does not hold, it means that they lie in the same chromosome (linked genes). From this observation begins the determination of the topography of the genes in the chromosomes. It is based on the fact that, during the maturation of the sexual cells or gametes, the series of genes contained in a chromosome behaves as an alignment that often exchanges part of its own elements with those of another, called homologous. For example, if in one chromosome are contained the genes for "yellow seed" and "wrinkled seed," in its homologue stand those for "green seed" and "smooth seed." During the maturation of the gametes they reconstitute the exchanged associations: "green seed" with "wrinkled seed" and "yellow seed" with "smooth seed." The more frequent the exchange (or crossing-over), the greater is the distance between the genes themselves in the chromosome.
It has been possible to demonstrate that the genes that are exchanged most frequently are farther apart from one another, while those that are inherited most frequently associated are closer. On this observation it has been possible to construct the so-called "map," that is, the graphic representation of the relative locations among the genes. From these one derives a fairly approximate idea of the structure of the genome, which turns out to be composed of a great number (hundreds or even thousands) of genes, aligned but probably not equidistant, each acting on one moment of the formation of one or more organs. These maps are known for various species of Drosophila, less completely for the mouse, the rabbit, the chicken, among animals; for maize, the convolvulus, the sweet pea, the snapdragon, etc., among plants. Very recent research is already beginning to orient us in the lower organisms such as phycomycetes, saccharomycetes and bacteria. Very useful were the checks of the maps by means of the study of structural aberrations of the chromosomes. It was thus possible to have also an idea (admittedly still very imprecise) of the order of magnitude of the real distances and of the dimensions of the genes. Today it is held that the unit of measure of such distances is one ten-thousandth of a mm. The structure of the gene, as an isolated entity, is however the object of another direction of genetics, radiogenetics, which flourished above all through the merit of H. Muller and N. W. Timoféeff-Ressowsky, by means of the determination of the frequency with which genes mutate (v. MUTAZIONE) following bombardment with ionizing particles (e.g., X-rays), with ultraviolet light or other physical agents. The results of this research led to elaborating a structural model of the gene, conceived as a macromolecule, or a small group of protein macromolecules.
The relations between genotype and the manifestation of characters are the object of a particular branch, which takes the name of phenogenetics. It avails itself of methods derived from experimental embryology and from biochemistry, and has already established some features of the laws that govern the actions exerted by the genes.
It is held that every gene, at a typical moment of development, controls one moment of the development of a given organ: in some better-studied cases (Drosophila and certain butterflies) it has been demonstrated that the control over development is exercised by certain genes at least by means of the production of stimulating substances similar to hormones (especially in the production of the pigments that color the eyes of Drosophila).
A problem that was once considered separate, but today is preferably seen as a complex of the manifestation of characters, of which one can study both the genotypic determination and the phenogenetics, is that of sex.
The comparative study of hereditary patrimonies, both in their gene structure and in their microscopically visible envelope or chromosomal arrangement (cytogenetics), has provided the bases for a genetic theory of the origin of species and of evolution.
Given that species can be considered connected by relationships of derivation, it is obvious that derivation must be understood as a complex of transformation of hereditary characters. Through theoretical research of a mathematical character and — more recently — experimental research on suitable material (not only drosophilas, but also plants of various genera), it has been possible to envisage that the principal elements that have governed and still govern the transformation of species are mutations and selection. Today some very clear cases are known of transitional forms between species, or of species that arose through known genetic phenomena (certain drosophilas, the genus galaeopsis among plants, etc.). If, however, there is general agreement among specialists in holding that the varieties into which species gradually differentiate are the germs of new species, and if the known genetic phenomena represent a strong support for the theory of evolution, the possibility of explaining in genetic terms, with currently known phenomena, the whole evolution of organisms is still debated.
Bibliography5 works
Claudio Barigozzi
END OF THE FIFTH VOLUME