. The physical basis of heredity . X from either parent, he is pure for whiteeyes in the sense that he has an X bearing the factor forwhite eyes and a Y that bears no factor making this F^ cross is exactly like a back-cross of a hetero-zygous female to a recessive male, and gives the samenumerical result, viz., 1:1. Cases of sex-linked inheritance of this kind are alsoknown in man. Color blindness in man appears to followexactly the same procedure as sex-linked inheritance in thevinegar fly—at least certain kinds of color blindness havebeen shown to do so. Haemaphilia also is sex-lin
. The physical basis of heredity . X from either parent, he is pure for whiteeyes in the sense that he has an X bearing the factor forwhite eyes and a Y that bears no factor making this F^ cross is exactly like a back-cross of a hetero-zygous female to a recessive male, and gives the samenumerical result, viz., 1:1. Cases of sex-linked inheritance of this kind are alsoknown in man. Color blindness in man appears to followexactly the same procedure as sex-linked inheritance in thevinegar fly—at least certain kinds of color blindness havebeen shown to do so. Haemaphilia also is sex-linked, andthere are four or five other defects in man that appearto come under this head. According to several accountsthere is an unpaired sex-chromosome (or two of them)in man, which is also called for by the genetic evidencerelating to sex-linkage in man, but since the female numberof chromosomes in man is stated by Guyer to be 24, andby von Winiwarter to be 48, it is unsafe as yet to appeal SEX-CHROMOSOMES AND INHERITANCE 171. Fia. 71.—Cross between a yellow, white-eyed female and a wild-tjpe (gray),red-eyed male. 172 PHYSICAL BASIS OF HEREDITY to this evidence as showing the identity of the sex-deter-mining mechanism of man and the vinegar fly. When two or more sex-linked characters are involvedat the same time, the situation is different only in so faras crossing over may take place in the female. It will besimpler to consider such a cross and its reciprocal in thereverse order from that just given. If a female withyellow wings {yy) and white eyes {wiv) is crossed to awild-type male, gray wings (Z) and red eyes {W), thesons are yellow white and the daughters are gray red(Fig. 71). When these are inbred there are four typesin Fz (ignoring sex), viz., the two original combinationsyellow white and gray red, and the two crossover com-binations yellow red and gray white. They occur in thefollowing ratios: Yellow white Gray red Yellow red Gray white 99 per cent. 1 per cent. I
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