PHENOMENA OF INHERITANCE 



433 



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 F t females of 

 this cross is bred with a white-eyed male half of the females of F 2 are 



Flies 



Chromosomes 



Gametes 



XX XI X( 



9 9 <J 



Fig. 61. Sex-linked Inheritance of White and Red Eyes in Drosophila. Pa- 

 rents, white-eyed male and red-eyed female ; Fi, red-eyed males and females ; F2, red- 

 eyed females and eaual 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 F x generation are red eyed and all the males white 

 eyed ("criss-cross" inheritance) and if these are interbred there are 

 produced in the F 2 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 con- 

 dition in man known as licvmopliilia, which is characterized by a defi- 

 ciency 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 



VOL. lxxxv. — 30 



