LINKAGE 



381 



somes they are sex-linked characters. 1 So far as is known, the Y- 

 chromosome of the male carries 110 sex- or sex-linked factors. This 

 general interpretation is directly applicable to the reciprocal cross (white- 

 eyed male X red-eyed female), in which, however, the relative proportions 

 of red-eyed and white-eyed flies in F\ and F 2 are different: in FI all flies 

 of both sexes have red eyes, while in Fz all the females and one-half of 

 the males have red eyes, white eyes appearing only in one-half of the 

 males. (See Morgan et al. 1915, pp. 16-20; Babcock and Clausen 1918, 

 pp. 74-77.) 



FIG. 146. Sex-linkage in Drosophila. Three successive generations at left; red eyes 

 shown in black. The history of the sex-chromosomes through these generations shown at 

 right; X -chromosome of original male shown in black. (Adapted from Morgan.) 



In such cases as the above it is evident that characters other than 

 sex may be referred to certain chromosomes of the complement: it is 

 possible not only to tell which chromosomes have to do with sex, but 

 also to identify the ones concerned in the production of red and white 

 eye colors. A large number of such sex-linked characters have been 

 identified in Drosophila, and several have been found in other animals. 

 Human colorblindness is a character which is inherited in a manner 

 analogous to that of sex-linked characters in Drosophila, and its me- 

 chanism is apparently the same. The presence of this defect more 

 commonly in men than in women, and its appearance in so few individ- 

 uals in affected lines, are due to the fact that it is both a recessive and a 



1 Sex-linked characters are not to be confused with sex-limited characters. The 

 latter are those found exclusively in one sex, and are now referred to as secondary 

 sexual characters. 



