MENDEL'S LAW, the law of heredity dis covered by Johann Gregor Mendel (q.v.) and first published in 1865 under the title Versuche fiber Pflanzenhydriden° in the Proceedings of the Society of Natural History of Briinn, Aus tria. Mendel made prolonged experiments in crossing varieties of the pea (Pisum sativum). His paper was overlooked until attention to his remarkable results was called by De Vries in 1900; he and also Correns and Tschermak at the same time independently rediscovered Men del's law. Mendel selected seven pairs of characters, such as The shape of the ripe seed, of the cotyledons, of the seed-pod, color of the seed-skin, length of stem, etc. Large numbers of crosses were made between peas differing in respect of one of each of these pairs of characters. It was found, says Bateson, that in each case the offspring of the cross exhibited the character of one of the parents in almost undiminished intensity, and which could not be at once referred to one or other of the parental forms were not found. "In the case of each pair of characters there is thus one which in the first cross prevails to the exclusion of the outer." This prevailing char acter Mendel called the dominant character, and to the other he gave the name of recessive character.
This law of dominance has been found by Bateson and by Castle to apply to animals as well as plants, and thus is a most important biological law. Thus when mating occurs be tween two organisms, whether vegetable or ani mal, differing in some character, the offspring frequently all exhibit the character of one parent only, in which case that character is said to he "dominant.° For example, on crossing white mice with gray mice, Castle found that the offspring are gray, that color-character be ing dominant. The character which is not seen in the immediate offspring is called recessive. for though unseen it is still present in the young, white in the experiment being the re cessive color.
The law of dominance has its exceptions: the hybrid often possesses a character of in own, instead of the pure character of on parent, as is true in cases of complete domi nance. The hybrid form often resembles supposed ancestral condition, when it is usually regarded as a reversion. Examples are the gray hybrid mice, which are indistin in appearance from the house mouse; also slate colored pigeons resulting from crossing whit< with buff pigeons.
One result of Mendel's discovery is the purity of the germ-cells. As stated by Castle "The hybrid, whatever its own character, pro duces ripe germ-cells which hear only the pure character of one parent or the other.' To breeders Mendel's law is of great isisportaixt because, as remarked by Castle, it reduces to an exact science the art of breeding in the case most carefully studied by him, that of en tire dominance. "No animal or plant is "pure' simply because it is descended from a long line of ancestors possessing a desired combination of characters, hut any animal is pure if it pm duces gametes (germ-cells) of only one sort. even though its grandparents may among them selves have possessed opposite characters.' The bearings of Mendel's discovery, coo firmed by De Vries' experiments, on the origin of species are most important. The Probkiz is whether aberrations, sports or discontinuous variations may not sometimes result in the formation of new species and types, or whether species are all the result of slow, continuous variations. As stated by Castle, *A sport bar ing once arisen affecting some one character of a species, may by crossing with the parent form he the cause of no end of disintegrative on the part of any or all of the characters of the species, and the disintegrated characters may, indeed must, form a great variety of new combinations of characters, some of which will prove stable and self-perpetuating.
Mendel's discoveries also explain the prin ciple that new types of organisms are extremely variable, whereas old types are subject to little variation. A new type which has arisen as a sport will cross with the parent form. The off spring, says Castle, will then inherit some dom inant character, others latent, and this will re sult in polymorphism of the race. Thus the suggestion of Calton that species may arise from sports is confirmed, while added cases are afforded by the recent remarkable experiments of De Vries, resulting in the origination of seven new species of primrose by sudden vari ations, or what he calls "mutations)) Consult Bateson, W., (Mendel's Principles of Heredity' (Cambridge 1913); Castle, W. E., 'Mendel's Law of Heredity' (Cambridge 1903) ; Morgan, T. H., 'The Mechanism of Mendelian Hered ity' (New York 1915). See HEREDITY; SEX.