PHENOMENA OF INHERITANCE 275 



Another case of sex-linked inheritance is 

 found in an abnormal condition in man known 

 as haemophilia, which is characterized by a de- 

 ficiency in the clotting power of the blood, and 

 consequently by excessive bleeding after in- 

 jury. "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 man- 

 ner of inheritance of this character is exactly 

 similar to the inheritance of white eyes in 

 Drosophila and is in all probability due to 

 similar causes. 



One of the most striking cases of sex-linked 

 inheritance is that form of color blindness 

 known as Daltonism, in which the affected 

 person is unable to distinguish between red 

 and green. It is known that males are more 

 frequently affected than females, and that 

 color blindness is in some way associated with 

 sex. It requires two determiners for color 

 blindness, one from the father, the other from 

 the mother, to produce a color blind female, 

 whereas only a single determiner is necessary 



