BIOTYPE. - In genetics, a b. is understood as a complex of individuals identical to one another in their endowment of hereditary characters. This concept arose from the investigations of Johannsen (1903), who, by propagating bean plants 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 endowed with an identical hereditary complement; examining various characters statistically (such as, for example, the weight and dimensions of the seeds), he noticed that these showed different mean values for each pure line, which remained constant in subsequent generations as long as the different pure lines were not crossed with one another. In a similar way Jennings (1908) in certain protozoa (Paramaecium) selected, by asexual reproduction (fission), a certain number of clones, or groups of animals which, all descending from one and the same parent, were perfectly homogeneous from the hereditary point of view; the characters of these clones likewise had characteristic and constant mean values. Pure lines (obtainable in plants or animals endowed with sufficient hermaphroditism) and clones (obtainable instead in organisms that reproduce asexually or by parthenogenesis) are so many 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 endowments of the two parents, frequently belonging to different b. Selection, however practiced, can separate the various b. from one another, and they remain disjoined if further heterogeneous crossings are avoided; but within a single b. selection no longer has any effect. From the systematic point of view, the b. can be considered the smallest unit of subdivision of an animal or plant species, whose races or varieties are also almost always mixtures of b.