GENETICS. – Genetics is defined as the science of heredity, that is, the doctrine of the phenomena concerning the reappearance of the same character in generations.
The primary objective of genetics is the knowledge of the material substrate determining hereditary characters, which are thus conceived as the result of an action originating from within the organism itself. This material substrate is generically called the hereditary patrimony or genotype: it is demonstrated that it repeats itself, more or less identically, in all the cells of the organism, and resides chiefly in the nucleus. The nuclear hereditary patrimony constitutes the genome, which in turn is composed of a large number of corpuscular units, called genes (v.), aligned in the chromosomes.
Genetics was formed around the laws of hereditary transmission in crosses, discovered by G. Mendel in 1865. They can be synthetically expounded as follows.
First law: if two individuals differing in a character (e.g., peas with green seeds – peas with yellow seeds) are taken from strains composed of identical individuals (strain with green seeds – strain with yellow seeds), a generation is obtained that is entirely uniform with only one of the two characters (in our case, yellow seeds).
Second law: if two individuals with yellow seeds are crossed, but are the offspring of yellow-seeded and green-seeded parents (i.e., two siblings from the generation referred to in the first law), a mixed progeny is obtained with 1/4 green seeds and 3/4 yellow seeds. The parents of this generation are called hybrids and heterozygous, while those of the previous generation (first law) are called pure or homozygous.
Third law: if individuals differing in more than one character 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 other according to the first two laws. Mendel’s three laws remained unknown until 1900, when De Vries, Tschermak, and Correns rediscovered them, confirming them on various botanical materials. Since then, Mendel’s laws have become the foundation upon which genetics has been built, so that the study of hereditary phenomena said to be Mendelian—i.e., those that follow Mendel’s laws—cannot be distinguished from the development of genetics itself. Only some additions have been made to them. Thus, it is now documented that dominance and recessiveness (first law) are often replaced by intermediate inheritance (pink flowers, from red flowers X white flowers). Moreover, the third law today has a very limited value, though it has not lost its importance. When it occurs, it means that the characters in question are due to genes located on different chromosomes (v. chromosome). When the third law does not occur, it means that they are located on the same chromosome (linked genes). From this observation stems the determination of the topography of genes in chromosomes. It 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 part of its elements with those of another, called homologous. For example, if a chromosome contains the genes for “yellow seed” and “wrinkled seed,” its homologue 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 that exchange most frequently are farther apart, while those that are more frequently inherited together are closer. Based on this observation, the so-called “map” has been constructed, i.e., the graphic representation of the relative positions of the genes. From these, a fairly approximate idea of the structure of the genome is obtained, which is composed of a large number (hundreds or even thousands) of genes, aligned but probably not equidistant, each acting on a moment in the formation of one or more organs. These maps are known for several species of Drosophila, less completely for the mouse, rabbit, chicken, among animals; for maize, convolvulus, sweet pea, snapdragon, etc., among plants. Very recent research is already beginning to shed light on lower organisms such as fungi, yeasts, and bacteria. The maps were very usefully verified by studying structural aberrations of the chromosomes. In this way, it was also possible to gain an idea (albeit still very imprecise) of the order of magnitude of the actual distances and the dimensions of the genes. Today, the unit of measurement for such distances is believed to be the ten-thousandth of a millimeter. The structure of the gene as an isolated entity, however, is the subject of another branch of genetics, radiogenetics, which flourished especially through the work of H. Muller and N. W. Timoféeff-Ressovsky, by determining the frequency with which genes mutate (v. mutation) following bombardment with ionizing particles (e.g., X-rays), ultraviolet light, or other physical agents. The results of this research led to the elaboration of a structural model of the gene, conceived as a macromolecule, or a small group of protein macromolecules.
The relationship between genotype and the manifestation of characters is the subject of a particular branch, called phenogenetics. It employs methods derived from experimental embryology and biochemistry, and has already established some features of the laws governing the actions expressed by genes.
It is believed that each gene, at a typical moment of development, controls a moment in the development of a given organ: in some better-studied cases (Drosophila and some butterflies), it has been demonstrated that the control over development is exerted by some genes at least through the production of hormone-like stimulating substances (especially in the production of pigments that color the eyes of Drosophila).
A problem that was once considered separately, but is now preferably seen as a complex of character expression, whose genetic and phenogenetic determination can be studied, is that of sex.
The comparative study of hereditary patrimonies, both in their genic structure and in their microscopically visible envelope or chromosomal arrangement (cytogenetics), has provided the basis for a genetic theory of the origin of species and of evolution.
Given that species can be considered as connected by derivation relationships, it is obvious that derivation must be understood as a complex of transformations of hereditary characters. Through theoretical research of a mathematical nature and, more recently, through experimental research on suitable material (not only Drosophila, but also plants of different genera), it has been possible to envisage that the main elements governing and regulating the transformation of species are mutations and selection. Today, some very clear cases of transitional forms between species, or of species arising from known genetic phenomena, are known (some Drosophila, the genus Galeopsis among plants, etc.). If, in general, there is agreement among specialists in considering the varieties in which species gradually differentiate as the germs of new species, and if known genetic phenomena provide strong support for the theory of evolution, the possibility of explaining the entire evolution of organisms in genetic terms with currently known phenomena is still debated.