Page:Popular Science Monthly Volume 85.djvu/437

Rh red eyes while half of the males have red eyes and the other half have white eyes (Fig. 61). On the other hand, if one of the F1females of this cross is bred with a white-eyed male half of the females of F2 are

 Drosophila. Parents, white-eyed male and red-eyed female; F1, red-eyed males and females; F2, red eyed females and equal numbers of red-eyed and white-eyed males. The distribution of sex chromosomes is shown to right of flies; X carries the factor for red eyes, X the factor for white eyes, O stands for absence of X. (After Morgan.)

red eyed and half are white eyed, and half of the males are red eyed and half are white eyed.

If now one of these white-eyed females is bred with a red-eyed male all the females of the F1 generation are red eyed and all the males white eyed ("criss-cross" inheritance) and if these are interbred there are produced in the F2 generation equal numbers of red-eyed and white-eyed males and females (Fig. 62).

The distribution of the maternal and paternal sex chromosomes (X) exactly parallels this distribution of this sex-linked character, as is shown in the right half of each of the figures, 61 and 62, and this is certainly very strong evidence that the differential factors for these characters are carried in these chromosomes.

Another case of sex-linked inheritance is found in an abnormal condition in man known as hæmophilia, which is characterized by a deficiency in the clotting power of the blood, and consequently by excessive bleeding after injury. "Bleeders" are almost always males, though the defect is always transmitted to a son from his mother, who does not usually show the defect because it appears in females only when both parents were affected. The manner of inheritance of this character is exactly similar to the inheritance of white eyes in Drosophila and is in