The Cellular Basis 183 



acter, as is shown in the right half hoth of Fig. (>i and of Fig. 

 62, and this suggests that the differential factors for these charac- 

 ters are carried in these sex chromosomes. 



By a series of ingenious experiments Foot and Strobell have 

 shown that the differential factors for certain sex-limited char- 

 acters in insects, that is, characters which are limited to one sex, 

 are not contained in the "sex chromosomes,'" and they argue that 

 the differential factors for sex and for sex-linked characters can- 

 not be located in these chromosomes. Morgan does not admit 

 the validity of their conclusions, since these apply only to sex- 

 limited and not to sex-linked characters. However these experi- 

 ments may be interpreted, there is good evidence that the factors 

 for the determination of sex and of sex-linked characters are dis- 

 tributed in the same way as the "sex chromosomes" are, and it 

 would be a surprising thing if these two phenomena should be 

 found not to be related causally. 



Haemophilia. — Another case of sex-linked inheritance is found 

 in an abnormal condition in man known as haemophilia, which is 

 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- 



