[Illustration: FIG. 60. Diagram of sex determination in type with XX female and XO male (after Wilson).] In man there are several characters that show exactly this same kind of inheritance. Color blindness, or at least certain kinds of color blindness, appear to follow the same scheme. A color blind father transmits through his daughters his peculiarity to half of his grandsons, but to none of his grand-daughters (fig. 38A). The result is the same as in the case of the white eyed male of Drosophila. Color blind women are rather unusual, which is expected from the method of inheritance of this character, but in the few known cases where such color blind women have married normal husbands the sons have inherited the peculiarity from the mother (fig. 38B). Here again the result is the same as for the similar combination in Drosophila. [Illustration: FIG. 61. Spermatogenesis in man. There are 47 chromosomes (diploid) in the male. After reduction half of the sperm carry 24 chromosomes (one of which is X) and half carry 23 chromosomes (no X).] In man the sex formula appears to be XX for the female and XO for the male (fig. 60), and since the relation is essentially the same as that in Drosophila the chromosome explanation is the same. According to von Winiwarter there are 48 chromosomes in the female and 47 in the male (fig. 61). After the extrusion of the polar bodies there are 24 chromosomes in the egg. In the male at one of the two maturation divisions the X chromosome passes to one pole undivided (fig. 61, C). In consequence there are two classes of sperms in man; female producing containing 24 chromosomes, and male producing containing 23 chromosomes. If the factor for color blindness is carried by the X chromosome its inheritance in man works out on the same chromosome scheme and in the same way as does white eye color (or any other sex linked character) in the fly, for the O sperm in man is equivalent to the Y sperm in the fly. In these cases we have been dealing with a single pair of characters. Let us now take a case where two pairs of sex linked characters enter the cross at the same time, and preferably a case where the two recessives enter the cross from the same parent. If a female with white eyes and yellow wings is crossed to a wild male with red eyes and gray wings (fig. 62), the sons are yellow and have white eyes and the daughters are gray and have red eyes. If two F_1 flies are mated they will produce the following classes. [Illustration: FIG. 62. Cross between a white eyed, yellow winged female of D. ampelophila and a red eyed, gray winged male. Two pairs of sex linked characters, viz., white-red and yellow-gray are involved. (See text.)] Yellow Gray Yellow Gray White Red Red White ------------ ------------- | | 99.% 1.%