HEREDITY IN SOME ANIMALS Mammals.—The character which has been most extensively studied is colour. This is inherited on very similar lines in the dog, cat, rabbit, guinea-pig, rat and mouse. The horse somewhat resembles these, but the cow, pig and sheep diverge considerably. Albinism is always recessive, though dominant genes producing black-eyed whites are often found. Where the wild colour is grey with a light-coloured belly, blackness is usually recessive, but again dominant genes converting grey into black are known in some species. Yellow is usually recessive to darker colours, and the brindled condition is often due to a multiple allelomorph inter mediate in its action between the genes determining yellow and full colour. Chocolate pigmentation, as in the liver spaniel, is always recessive to black. Piebaldness may be recessive or domi nant, and different genes may affect the pattern differently. Thus the white face of Hereford cattle is due to a dominant gene, while the ordinary type of piebaldness is neither dominant nor recessive, heterozygotes being intermediate.
Some structural characters are inherited on Mendelian lines. Thus long hair is recessive in rabbits, guinea-pigs, dogs and cats; and the polled character is dominant in cattle. A number of mal f ormations and lethal genes are recessive. But the most econom ically important characters, such as butter- and milk-yield in cattle, are generally due to several genes. These are inherited through both sexes, even when only one manifests the character. Thus the Danish dairy industry owes much to two "butter bulls" who transmitted a gene or genes determining high butter yield to all their cow calves. In the large domestic animals selection is mainly practiced on the males, and these can of ten be genetically analysed by testing with a recessive. Thus a black polled bull who gives only black polled calves with red-horned cows must be homozygous for both the polled and black genes, and will yield only polled black calves even with heterozygous cows.