8 4 



ZOOLOGY 



Sex-linked 

 and sex- 

 limited 

 characters 



living may differ, even when the zygotes formed are 

 half of each sex. Cases are known among insects, in 

 which the sperms of the male-producing type degener- 

 ate, so that only females are produced. Unfertilized 

 eggs, developing parthenogenically, give rise to males. 

 The quantitative difference between the sexes is thus 

 maintained. Should it happen, in any case, that only 

 part of the sperms degenerate or fail to function, the 

 sex ratio will be disturbed. 



6. Certain characters are said to be sex-linked. 

 These are not the secondary sexual characters (sex- 

 limited), and have no necessary connection with any of 

 the sexual activities. Sex-linked characters are those 

 for which the determiners are carried by the sex chro- 

 mosome (^-chromosome). How can such a fact be 

 ascertained, since, although the chromosome may be 

 seen, no one can distinguish determiners in it ? In the 

 Drosophila flies, the normal color of the eyes is bright 

 red. A variation with white eyes appeared, and Pro- 

 fessor Morgan proved experimentally that it was sex- 

 linked, in the following manner : A white-eyed female, 

 mated with a red-eyed male, gave only red-eyed females 

 and white-eyed males. These, crossed together, gave 

 both red- and white-eyed of each sex. The theory is 

 as follows : Red-eye and white-eye are allelomorphic, 

 that is, paired opposites in inheritance. Red is 

 dominant over white. If these determiners are in the 

 ^-chromosome, then the white-eyed female has two 

 "white" x's. The red-eyed male has one "red" x. 

 Half the sperms of the male carry the "red" x, and pro- 

 duce females carrying one "red" and one "white" x. 

 Red being dominant, such heterozygous females are 

 red-eyed. The other half of the sperms carry no x, and 

 unite with gametes from the females carrying one 



