Heredity, Cytoplasmic

(extranuclear, nonchromoso-mal, or plasmatic heredity), continuity in the material structures and functional properties of the organism, which are determined and transmitted by factors located in the cytoplasm. The aggregate of these factors (plasmagenes, or extranuclear genes) constitutes a plasmon, which is analogous to the genome, the set of chromosomal genes. Plasmagenes are found in the self-reproducing cell organelles—the mitochondria and plastids, including chloroplasts.

Deviations from Mendelian segregation observed after crosses are the main evidence for cytoplasmic heredity. The cytoplasmic elements carrying the plasmagenes separate in the daughter cells in an irregular manner, rather than in the orderly fashion characteristic of the chromosomal genes. Plasmagenes are transmitted mainly through the female sex cell (egg cell), because the male sex cell (sperm) contains almost no cytoplasm. (This, however, does not prevent the transmission of plasmagenes through male gametes.) Therefore, cytoplasmic heredity is studied by means of special crosses in which a given organism (or group) is used both as a maternal form and as a paternal form (reciprocal cross).

In plants and animals the differences determined by cytoplasmic heredity are primarily the dominance of maternal traits and the expression of a particular phenotype when a cross is made in one direction and the loss of that phenotype when the direction of the cross is changed. Cytoplasmic male sterility transmitted through the female line is widely used to obtain hybrid heterotic forms of some agricultural crops, particularly corn. Cytoplasmic heredity may also be studied by the transplantation of the nucleus from one cell to another.

Cytoplasmic heredity is distinguished from infective heredity —that is, the transmission through the cytoplasm of symbiotic or mildly pathogenic self-reproducing particles that are not normal cell components necessary for metabolic activity (in the fruit fly Drosophila, the sigma virus, which attacks plastids; in para-mecia, the rickettsia-like kappa particles). In all cases studied, plasmagenes are, from the chemical standpoint, the DNA molecules that are found in many self-reproducing organelles. (A plasmagene may contain several tenths of a percent of the total cellular DNA.)

Although the carriers of plasmagenes have a certain characteristic degree of genetic autonomy, they are also subject to control by the chromosomal genes. Some mutations are induced in plastids by nuclear genes, which also have some control over the functioning of the plastids. In addition, the quantity of DNA in mitochondria has been found to be insufficient to carry all the necessary information about their functions and structure. Thus, the structure of mitochondria is also determined, at least in part, by the genome. Nuclear and extranuclear genes may also interact in the realization of the phenotype.