Page:Popular Science Monthly Volume 85.djvu/341

Rh again and again, so that there is no doubt that they constitute an important rule of inheritance among all organisms.

In brief the "Mendelian Law of Alternative Inheritance" or of hereditary "splitting" consists of the following principles:

(a) The principle of unit characters.—The total heritage of an organism may be analyzed into a number of characters which are inherited as a whole and are not further divisible; these are the so-called "unit characters" (de Vries).

(b) The principle of dominance.—When contrasting unit characters are present in the parents they do not as a rule blend in the offspring, but one is dominant and usually appears fully developed, while the other is recessive and temporarily drops out of sight.

(c) The principle of segregation.—Every individual germ cell is "pure" with respect to any given unit character, even though it come from an "impure" or hybrid parent. In the germ cells of hybrids there is a separation of the determiners of contrasting characters so that different kinds of germ cells are produced, each of which is pure with regard to any given unit character. This is the principle of segregation of unit characters, or of the "purity" of the germ cells. Every sexually produced individual is a double being—double in every cell—one half having been derived from the male and the other half from the female sex cell. This double being, or zygote, again becomes single in the formation of the germ cells only once more to become double when the germ cells unite in fertilization.