GENETICS, a term coined by Bateson to designate that portion of biology concerned with heredity, variation, develop ment and evolution. It is the science which seeks to account for the resemblances and the differences which are exhibited among organisms related by descent. Its problems are those of the cause, the material basis, and the method of maintenance of the speci ficity of germinal substance; in other words, "how the characters of parents and offspring are related, how those of the adult lie latent in the egg, and how they become patent as development proceeds." Its methods are those of observation, experimental breeding, cytology and experimental morphology. Its prosecution demands a knowledge of general physiology and of mathematics. It has both scientific and practical application : its principles impinge upon all doctrines of evolution and upon agricultural, animal and plant breeding practices. Its possible applications to human affairs have created the need for and the development of the applied science of eugenics (q.v.).
Out of the accumulated facts of genetical experimentation, there has been developed the theory of the gene (q.v.) intended to accommodate these facts. It states (I) that the hereditary characters of the individual are referable to paired elements (the genes) in the germinal material (the chromosomes, q.v.) which are held together in a definite number of linkage groups; (2) that the members of each pair of genes separate when the germ-cells mature in accordance with Mendel's first law, and that in con sequence each ripe germ-cell comes to contain one set only; (3) that the members of different linkage groups assort inde pendently in accordance with Mendel's second law; (4) that an orderly interchange—crossing-over—also takes place, between the elements in corresponding linkage groups; and (5) that the frequency of crossing-over furnishes evidence of the linear order of the genes in each linkage group and of the relative position of the genes with respect to each other.
The gene, a conception as reasonable and as real as the atom, is to be looked upon as a particular state of organization of the chromatin at a particular point in the length of a particular chromosome. (See ANIMAL BREEDING, HEREDITY, CYTOLOGY, MENDELISM and PLANT BREEDING.) BIBLIOGRAPHY.-T. H. Morgan, The Theory of the Gene (bibl., Bibliography.-T. H. Morgan, The Theory of the Gene (bibl., 1926) ; F. A. E. Crew, Organic Inheritance in Man (1927). B. Bateson, William Bateson, Naturalist (1928). (F. A. E. C.)