The inheritance of sex has been one of the great biological discoveries of the present cen tury. The factor or factors of sex have been shown to be carried by special chromosomes called the sex chromosomes. In certain great groups (mammals, most insects, etc.) the occur rence of two of these chromosomes, called the X chromosomes, produces a female, the pres ence of one of them produces a male. Thus the female is XX, the male X. At the reduc tion division in the female one X is eliminated from the egg so that each egg contains but one X chromosome. In the male there is only one X, this X chromosome goes in the reduction division to only one sperm cell of the two formed, so that two classes of spermatozoa re sult. At the time of fertilization random meet ing of any egg with any spermatozoon will give two classes of individuals, those with two X's (females) and those with one X (males). This mechanism preserves the numerical equality of the sexes. In other groups (birds, moths) the relation is reversed, the male containing two X's, the female one; hence all of the sperma tozoa contain one X; half of the eggs contain but one X, the other half none. This mechanism gives the same numerical results but the sex factors are presumably different. In hermaphroditic forms all of the individuals are alike in their chromosomal numbers, and the differentiation of testes in one part and ovaries in another part of the same individual must be determined by the same kind of relation that de termines the differentiation of any of the organs of the body. This is not the result of different chromosomal complexes but of the relation of the protoplasmic parts to each other.
In consequence of the method of distribu tion of the sex chromosomes a unique type of inheritance called "sex linked inheritance" has been recognized. The sex chromosomes carry many other factors besides the sex factor which follows the distribution of the sex chromosomes. Thus, in man certain kinds of color blindness and hemophilia (bleeding re sulting from failure of the blood to coagulate) are inherited as follows: an affected male transmits his characteristics to half of his grand sons but to none of his sons, daughters or granddaughters; but all of his daughters and half of his granddaughters carry his affected X chromosomes, and reproduce their peculiarity in half of their sons. Conversely, an affected
female transmits her peculiarity to all of her sons (since each gets her affected X chromo some) but to none of her daughters since the other X chromosome that they receive from their father counteracts the injurious effect of the maternal X chromosome, etc. In birds, where the female is X and the male XX, these relations are just reversed.
The word heredity has come to-day to have a more precise meaning than in the i past, be cause the recurrence of characters n succes sive generations is now known to come not from the body of the parent, but to be an ex pression of the factorial composition of the germ plasm. The germ plasm is the heritage of the species or race and changes only through mu tation in one or more of its elements or genes. The character of each individual that arises out of this germ plasm is determined by the sum total of the factors contained in the germ, which factors acting on the cytoplasm deter mine the character of the result in any given environment. Heredity is the name given to express this relation of continuity of the germ plasm material and its consequences in the suc cessive generations that arise from the germ plasm.
Bibliography.—Bateson, William, Principles of Heredity' (Cambridge 1913); Castle, William Ernest, (Genetics and Eu genics' (Cambridge, Mass., 1916) ; De Vries, Hugo, Mutation Theory' (Chicago 1909 10) ; De Vries, Hugo, 'Species and Varieties, their Origin by Mutation' (Chicago 1905); Morgan, Thomas Hunt, Mechanism of Mendelian Heredity> (New York 1915); Pearson, Karl, various articles in Biometrika; Weismann, Evolution Theory' (Eng. ed., 1906); Babcock and Clausen, 'Genetics in Re lation to Agriculture' (New York 1918).