19 Heredity and Environment 



no doubt that the factors for the determination of sex and sex- 

 linked characters are distributed in the same way as the "sex 

 chromosomes" are, and this proves that these factors are located 

 in the "sex chromosomes." 



Haemophilia. Another case of sex-linked inheritance is found 

 in an abnormal condition in man known as haemophilia, which i? 

 characterized by a deficiency in the clotting power of the blood 

 and consequently by excessive bleeding after injury. "Bleed- 

 ers" are almost always males, though the defect is always trans- 

 mitted 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 all 

 probability due to similar causes. 



Daltonism. One of the most striking cases of sex-linked in- 

 heritance 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 to 

 produce a color blind male, just as is true of sex. The accom- 

 panying diagrams (Figs. 63, 64) illustrate the method of inheri- 

 tance of color blindness. J[ represents the normal X-chromo- 

 some, Q its absence (or rather the F-chromosome since the sex 

 chromosomes of the human male are XY and not XOQ and X 

 the X-chromosome which carries the factor for color blindness. 



It will be seen that a color blind father and a normal mother 

 have only normal children, but the father transmits to his daugh- 

 ters and not to his sons the sex-determiner which carries the 

 factor for color blindness. But since color blindness does not 

 develop in females unless it is duplex (i.e. comes from both 

 father and mother) whereas it develops in males if it is simplex 



