Selection of plants and animals
A variety or breed is a population created by humans whose stable useful traits are inherited; each has a norm of reaction, so even better conditions do not raise productivity without limit. The main methods are selection and hybridisation. In mass selection (in cross-pollinated plants) a group of plants with the wanted traits is picked, and the variety is not genetically uniform; in individual selection offspring are taken from separate plants and pure lines are created. Hybridisation is within a species, between geographically distant forms (for example with wild cotton for the “Tashkent” cotton varieties) and between distant species: the cabbage × radish hybrid (G. D. Karpechenko, 1920s) was sterile and became fertile when its chromosome set was doubled (polyploidy); the wheat × rye hybrid is triticale. In artificial mutagenesis mutations are obtained with radiation or chemicals (only specialists work with mutagens, in laboratories). Heterosis is the greater yield, resistance and vigour of F1 hybrids compared with their parents. In animal selection offspring are few, so mainly individual selection and assessment of the exterior (outward build) are used; many productive breeds came from crosses between breeds and between distant forms (for example argali × merino — the arkhar-merino sheep). Today genome analysis of traits and genetic engineering have been added.
“Choose the method”. Cards: 1) picking the best ears from a field for seed; 2) crossing with a wild relative; 3) doubling the chromosome set; 4) growing offspring of one plant to make a pure line. Name the method for each.