234 HEREDITARY TRANSMISSION [CH. 



these will continue to produce sons and daughters that are 

 exceptions to the normal rule of sex-limited transmission. 

 For a female of constitution XXT will produce four kinds 

 of mature eggs, bearing XT, X, XX, and T, and of these the 

 XX and T eggs will produce exceptions to the rule of sex- 

 limited inheritance. BRIDGES has shown that XXT females 

 exist, and that in the strain in which they occur all the 

 kinds of exceptions that are to be expected theoretically are 

 actually found. Thus, as MORGAN writes, "a result that 

 seemed in contradiction with the chromosome hypothesis 

 has turned out to give a brilliant confirmation of that theory 

 both genetically and cytologically." It is possible, though 

 not yet fully proved, that other exceptions to the rule of 

 sex-limited transmission are also due to non-disjunction of 

 the sex-chromosomes either in the egg or in the spermato- 

 cytes. 



Before leaving the subject of the relation between sex- 

 limited transmission and the behaviour of the sex-chromo- 

 somes, a short reference should be made again to the fact 

 that in the male of Drosophila and in the female Silk-worm 

 no crossing-over takes place between coupled characters, 

 although it occurs regularly between these characters in the 

 opposite sexes. If this were true only of characters that 

 show sex-limited transmission, the fact would be easily ex- 

 plicable on the assumption that no crossing-over could take 

 place between unequally paired sex-chromosomes such as 

 exist in the male of Drosophila and probably (by inference) 

 in the female Silk-worm. But the absence of crossing-over 

 in the sex that shows sex-limited transmission is equally 

 conspicuous in characters that are not in any way sex- 

 limited, . and it must be concluded that the existence of an 

 unpaired or unequally paired sex-chromosome in some way 

 inhibits the crossing-over of coupled characters in all the 



