GENE

GENE. — It is the structural and functional unit of the hereditary material contained in the nucleus, that is, the smallest portion of living substance endowed with the capacity to determine a character. It was designated by this name by Johannsen (1911). Synonyms are “determinant” or “Mendelian factor.” The gene is a particle invisible under the ordinary microscope, conceived as a macromolecule 2000 m² in diameter. It is aligned with others, of similar dimensions, within a chromosome; the latter therefore contains a row of genes.

The position (locus) occupied by the gene in the chromosome is constant, except in the case of chromosomal mutations (v. MUTAZIONI). The gene controls the manifestation of several characters (the pleiotropic action of the gene), but in a more appre-

…scant of a single character; hence the practice of designating each g. by the character easiest to recognize (e.g., when one speaks of the g. for the yellow seed or the g. for the green seed, one means to recall the most conspicuous manifestation, disregarding others that are more difficult to identify and that appear in other organs). The g. has the capacity to reproduce itself by doubling before each cell division. In fact, when a cell multiplies by indirect division, or mitosis, through karyokinesis, the chromatin of the nucleus in which the g. reside is divided into two parts, each of which passes to one of the two daughter cells; nevertheless, the content in g. is identical in the two parts. Moreover, the g. changes, transforming its manifestations and its structure, but not its position on the chromosome, which is called facut. The various forms in which a g. may occur following mutation are called alleles or allelomorphs.

Alleles behave in crosses according to Mendel’s laws. For example, in Drosophila melanogaster, the g. for the white eye is nothing other than an allele of a g. that controls eye pigmentation and that, undergoing transformation through mutation, gives an allele of the former.

The g. are present in all cells. From them proceed chains of reactions that ultimately produce the manifested, or phenotypic, character: some of these reactions have been identified in the production of diffusible oligodynamic substances; they are known in insects.

In the cytoplasm of infusorians, particles vaguely comparable to g., called plasmagenes, were recently identified. They are not arranged in a line, but very little is known about them so far.