GENETICS. — Genetics is defined as the science of heredity, that is, the doctrine of phenomena concerning the reappearance of the same character in successive generations of a living organism, not directly induced by external causes. Genetics therefore does not include the study of characters induced by the persistence of environmental factors (nutrition, light, humidity, parasitism, etc.), which likewise recur in the succession of generations. This fundamental distinction assumes particular importance in the study of pathological characters, which are considered hereditary only when the cause of the character cannot be attributed to the aforementioned external factors: for example, syphilis is not considered hereditary but congenital, because it is due to infection transmitted during fetal life.
The first objective of genetics is 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 is generically called the hereditary endowment or genotype: it has been demonstrated that it is repeated, more or less identically, in all the cells of the organism, and resides chiefly in the nucleus. The nuclear hereditary endowment constitutes the genome, itself composed of a large number of corpuscular units, called genes (v.), aligned in the chromosomes.
Genetics arose around the laws of the heredity of crosses, discovered by G. Mendel in 1865. They may be summarized as follows.
First law: if two individuals differing in one character are taken (e.g., peas with green seeds and peas with yellow seeds) and deriving from strains composed of identical individuals (a strain with green seeds and a strain with yellow seeds), an entirely uniform generation is obtained, possessing only one of the two characters (in our case, yellow seed).
Second law: if two individuals with yellow seeds, but descended from yellow-seeded and green-seeded parents (that is, two siblings of the generation referred to in the First Law), are crossed with each other, a mixed offspring is obtained, with 1/4 green seeds and 3/4 yellow seeds. The parents of this generation are called hybrids or heterozygotes, whereas those of the preceding generation (First Law) are called pure or homozygous.
Third law: if individuals differing in several characters are crossed (e.g., two characters of the seed, yellow and green; two of the stem, tall and short, etc.), each is inherited independently of the others according to the first two laws. Mendel’s three laws remained unknown until 1900, when De Vries, Tschermack, and Correns rediscovered them and confirmed them using a variety of botanical material. From that time onward Mendel’s laws became the foundation upon which genetics was built, so that the study of hereditary phenomena known as Mendelian—that is, phenomena following Mendel’s laws—cannot be distinguished from the development of genetics itself. Only certain additions were made to them. Thus, it is now documented that dominance and recessivity (First Law) are often replaced by intermediate inheritance (pink flowers, from red flowers × white flowers). Moreover, the Third Law today has a very limited validity, although it has not lost its importance. When it occurs, it means that the characters considered are due to genes located on different chromosomes (v. CROMOSOMA). When the Third Law does not occur, it means that they lie on the same chromosome (linked genes). From this observation proceeds the determination of the topography of genes in the chromosomes. This is based on the fact that, during the maturation of sex cells or gametes, the series of genes contained in a chromosome behaves like an alignment that often exchanges some of its elements with those of another chromosome, called homologous. For example, if one chromosome contains the genes for “yellow seed” and “wrinkled seed,” its homolog contains those for “green seed” and “smooth seed.” During the maturation of the gametes, the exchanged associations are reconstituted: “green seed” with “wrinkled seed” and “yellow seed” with “smooth seed.” The more frequent the exchange (or crossing-over), the greater the distance between the genes themselves in the chromosome.
It has been demonstrated that genes which exchange most frequently are farther apart, whereas those more frequently inherited together are closer. On the basis of this observation it has been possible to construct the so-called “map,” that is, the graphic representation of the relative locations of the genes. From these maps one obtains a fairly approximate idea of the structure of the genome, which consists of a large number (hundreds or even thousands) of genes, aligned but probably not equidistant, each acting at a stage in the formation of one or more organs. These maps are known for several species of drosophila, and less completely for the mouse, rabbit, and chicken among animals; and for maize, bindweed, sweet pea, snapdragon, etc., among plants. Very recent research is already beginning to guide us in lower organisms such as phycomycetes, saccharomycetes, and bacteria. Checks on the maps through the study of structural chromosome aberrations proved highly useful. In this way it was also possible to gain an idea (in truth still highly imprecise) of the order of magnitude of the actual distances and of the dimensions of the genes. It is now believed that the unit of measurement of such distances is one ten-thousandth of a millimeter. The structure of the gene, as an isolated entity, is nevertheless the subject of another branch of genetics, radiogenetics, which flourished chiefly through the work of H. Muller and N. W. Timoféeff-Ressowsky, by determining the frequency with which genes mutate (v. MUTAZIONE) following bombardment with ionizing particles (e.g., X-rays), ultraviolet light, or other physical agents. The results of these investigations led to the elaboration of 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 subject of a particular branch, known as phenogenetics. It employs methods derived from experimental embryology and biochemistry, and has already established certain features of the laws governing the actions expressed by genes.
It is believed that each gene, at a characteristic stage of development, controls a stage in the development of a given organ: in certain better-studied cases (drosophila and some butterflies), it has been demonstrated that control over development is exercised by some genes at least through the production of hormone-like stimulating substances (especially in the production of the pigments that color the eyes of drosophila).
A problem that was once considered separately, but which today is preferably regarded as a complex of the expression of characters, whose genotypic and phenogenetic determination can both be studied, is that of sex.
The comparative study of hereditary endowments, both in their genetic structure and in their microscopically visible envelope or chromosomal constitution (cytogenetics), has provided the foundations for a genetic theory of the origin of species and of evolution.
Given that species may be regarded as connected by relations of descent, it is obvious that descent must be understood as a complex transformation of hereditary characters. Through theoretical research of a mathematical nature and—more recently—experimental research on suitable material (not only drosophila, but also plants of various genera), it has been possible to propose that the principal elements that have governed and continue to govern the transformation of species are mutations and selection. Today, several very clear cases are known of transitional forms between species, or of species arising through known genetic phenomena (certain drosophila, the genus galaeopsis among plants, etc.). If, however, specialists generally agree in maintaining that the varieties into which species gradually differentiate are the germs of new species, and if known genetic phenomena provide strong support for the theory of evolution, the possibility of explaining the whole evolution of organisms in genetic terms, using currently known phenomena, is still under discussion.
Claudio Barigozzi
END OF THE FIFTH VOLUME