229 



In the cross between the combinations Ab and aB, although 

 the offspring are identical with those from AB x ah, there is 

 an inversion of the ratios in which the factors are associated, 

 thus : 



etc. 



In animals it is not certain that the ratios 3:1.7:1, etc., are 

 followed. 



In no case of this kind is it certain that absolute coupling 

 occurs between the associated factors. Now there is very 

 strong evidence that the sex-dctcrmining factors behave as a 

 pair of Mendelian characters, and that sex-limited inheritance 

 is a special case of gametic coupling of the t\-pe illustrated ; 

 it is therefore of importance to determine whether it is absolute 

 or partial, as in the cases referred to. In pigeons and canaries 

 partial sex-limited inheritance is known to exist ; in Abraxas 

 and Drosophila there is as yet no certain evidence that the 

 couphng with the sex-factor is not absolute, although certain 

 exceptions which have been recorded point to the possibihty 

 of its existence. It has been suggested, however, that the dis- 

 tribution among the sexes of the forms taii and hc^i'its. in the 

 well-known experiments of Stan'DFUSS on Ag/ia Inn. indicated 

 partial sex-limitation in that species. 



2. A still more important theoretical question arises from 

 a comparison of Abraxas and Drosophila. In Abraxas it is 

 clear that the sex must be determined in the egg before ferti- 

 lisation, since the factor d is transmitted only to those eggs 

 which will become males. In Drosophila. however, the same 

 reasoning suggests that it must be the spermatozoon which 



