GENE. - It is the structural and functional unit of the hereditary patrimony contained in the nucleus, that is, the smallest portion of living substance endowed with the capacity to determine a character. It was given this name by Johannsen (1911). Synonyms are "determinant" or "Mendelian factor." The g. is a particle invisible to the ordinary microscope, conceived as a macromolecule 2000 m² in diameter. It is aligned with others of similar dimensions within a chromosome; this therefore contains a row of g.
The position (locus) that the g. occupies in the chromosome is constant, barring chromosomal mutations (v. MUTAZIONI). The g. controls the manifestation of several characters (pleiotropic action of the g.), but in a more appa-
and less apparent of a single character; hence the practice of designating each gene by the character easiest to recognize (for example, when one says: gene for the yellow seed or gene for the green seed, the intention is to recall the most conspicuous manifestation, disregarding others that are more difficult to identify, which appear in other organs). The gene has the faculty of reproducing itself by doubling before every cell division. In fact, when a cell multiplies by indirect division, or mitosis, with karyokinesis, the chromatin of the nucleus in which the genes reside divides into two parts that go to each of the two daughter cells, and yet the gene content is identical in the two parts. Furthermore, the gene mutates, transforming its own manifestations and its own structure, but not its own site in the chromosome, which is called the locus. The different ways in which a gene can present itself as a result of mutation are called alleles or allelomorphs.
Alleles behave in crosses according to Mendel's laws. For example, in Drosophila melanogaster the gene for the white eye is nothing other than an allele of a gene that controls the pigmentation of the eye and that, transforming itself by mutation, gives an allele of the former.
Genes are present in all cells. From them depart chains of reactions that come to produce the manifest or phenotypic character: part 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 genes, called plasmagens, were recently identified. They are not aligned, but very little about them is known so far.
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Claudio Barigossi