Epigenetics and the environment: emerging patterns and implications

Population genetics

1. The classification of genetics Genetics, the study of the hereditary mechanism, and of the rules by which heritable qualities are transmitted from one generation to the next, is a science sufficiently new for its subdivisions and their mutual relationships to be ill-defined, or, at least, in process of finding their definitions. Consequently, of the many adjectives which one hears and sees applied to the word ‘genetics’ some are meaningful, while many others imply some distinction scarcely worth making. The wide applicability of Mendel’s principles invites a classification based on the subject-matter, such as human genetics,Drosophila genetics, mouse genetics, plant genetics, and so on, but no distinction of importance can be made in this way. In recent years indeed, organisms much more different than the higher animals and green plants have been shown to exhibit closely analogous genetic phenomena; the list now includes Protozoa, fungi, bacteria and viruses, and to these obscure and difficult forms some of the finest of modern researches have been devoted (Sonnerborn 1947; Lederberg 1951). The very existence of sexual reproduction in many of these groups was only discovered, and can still only be demonstrated, by genetic methods. With such an enormous range of diverse forms, though all conforming to the same principles of inheritance first glimpsed by Gregor Mendel, the technical methods of culture differ greatly according to the breeding system current—self-fertilizing hermaphrodites, self-sterile hermaphrodites, animals and plants with separate sexes, ephemeral or long-living, capable perhaps of vegetative or of apomictic reproduction. In the microfungi heterocaryosis, exhibiting fusion of the cytoplasm without using the recombination mechanism of the nucleus, forms a branch of genetics which has been greatly forwarded by Pontecorvo at Glasgow, and by Mather at Birmingham.

The gene

The gene has sometimes been described as a purely idealistic concept, divorced from real things, and again it has been denounced as wishful thinking on the part of those too mechanically minded. And some critics go so far as to assert that there is not even such a thing as genetic material at all, as distinct from other constituents of living matter.