generation in biology

From Big Medical Encyclopedia

Understand group of individuals in population with identical degree of relationship in relation to parents as GENERATION, i.e. direct posterity of individuals of the previous generation. Sometimes instead of the term «generation» apply the concept «generation». Average life expectancy of P. corresponds to middle reproductive age of individuals of one look.

In demographic researches the concept «generation» is applied to designation of set of the people who were born in the same year; distribution of individuals according to P. forms age structure of society. The term «generation» is used also in historical, sociological and other aspects. In particular, in historical and sociological researches understand a community of the people connected not date of birth as P., but a community of any significant historical events (e.g., generation of the Great Patriotic War, post-war generation, etc.) * In P.'s biology closely connect with existence of the alternating stages life cycle (see) this or that of a type of organisms. P.'s alternation is characteristic of life cycles of many plants and animals. Distinguish primary and secondary alternation

of P. K to primary alternation of P. refer alternation sexless (a sporophyte at plants) and sexual (gametogophyte) of ways of reproduction. Alternation sexual with parthenogenetic or with vegetative ways of reproduction is referred to cases of secondary alternation of P. Razlichayut by two forms of secondary alternation of P. — a metagenesis and a heterogony. At metagenesis (see) vegetative and sexual ways of reproduction alternate, but at the same time both vegetative, and sexual P. is kept by an identical diploid set of chromosomes; the metagenesis is characteristic of coelenterates. Sexual and parthenogenetic P.'s alternation is called a heterogony (e.g., at rotifers, gallits-gall wasps, plant louses). At plant louses diploid egg develops in the spring in an adult wingless female of a plant louse, edge by diploid parthenogenesis (see) during the summer gives several generations of parthenogenetic females; at the end of summer from the eggs laid by parthenogenetic females there are winged females who produce eggs of two types with different number of gonosomes, but with diploid number of autosomes. From one type of eggs wingless diploid females, from eggs of other type — wingless diploid males develop in the fall. Males and this P.'s females produce haploid gametes, at merge to-rykh the diploid wintering egg giving in the spring rise to parthenogenetic generations of females is formed. P.'s alternation is characteristic of many parasites.

Knowledge of patterns of alternation of P. of different types of organisms allows to develop biol, methods of controlling with harmful, including with parasitic, animal species.

Bibliography: Gartman M. The general biology, the lane with it., M. — L., 1936; The Guide to cytology, under the editorship of A.S. Troshin, t. 2, page 359, M. — JI., 1966.

H. N. Vorontsov.