BIOTYPE. — By b. is meant, in genetics, a complex of individuals identical among themselves with respect to the constitution of hereditary characters. This concept arose from the investigations of Johannsen (1903), who, by having bean plants reproduce through self-fertilization, was able to isolate a certain number of so-called pure lines, that is, groups of plants descended from a single parent and therefore possessing the same hereditary constitution; by statistically examining various characters (such as, for example, the weight and dimensions of the seeds), he observed that these displayed different mean values for each pure line, which remained constant in successive generations so long as the different pure lines were not crossed with one another. Similarly, Jennings (1908), in certain protozoa (Paramaecium), selected, through asexual reproduction (fission), a certain number of clones, or groups of animals that, all deriving from the same parent, were perfectly homogeneous from the hereditary standpoint; the characters of these clones, too, had characteristic and constant mean values. Pure lines (obtainable in plants or animals possessing sufficient hermaphroditism) and clones (obtainable instead in organisms that reproduce asexually or by parthenogenesis) are each b., according to the definition given above.
Every natural population consists of many b., mixed together as a consequence of sexual reproduction, which is accompanied (in fertilization) by the fusion of the hereditary constitutions of the two parents, frequently belonging to different b. Selection, however practiced, can separate the various b. from one another, and they remain distinct if further heterogeneous crossings are avoided; but within a b. selection no longer has any effect. From the systematic point of view, the b. may be considered the smallest unit into which an animal or plant species is subdivided; its races or varieties, too, are almost always mixtures of b.