228 



a white-eyed female all the female offspring were red-eyed, all 

 the males white-eyed, thus— i? being red, r white (absence of 

 red) : 



But the converse cross, heterozygous red female by white male, 

 gave red and white in each sex, thus : 



Rr^ rr'^ Rr¿ rr¿ 



A number of other sex-limited characters have also been 

 discovered in this species by Morgan. 



Two questions of great theoretical importance arise from 

 these cases, both concerned with the nature of sex-determination. 



I. In both plants and animals (including Drosophila itself), 

 it is known that " coupling " may occur in the gametes between 

 factors for distinct characters, in such a way that characters 

 which are associated in the parent tend to be associated in the 

 gametes of the offspring. If A and B are two such characters, 

 a and b representing their absence, then it is found that when 

 an individual containing A and B is mated with one of con- 

 stitution ab, the gametes of the offspring show an excess of the 

 combinations AB and ah, and a deficiency of aB and Ah, and 

 the ratios in which they occur, in plants at least, show a certain 

 regularity, thus : 



Parents AB x ah 



1 

 Offspring A Bah 



Ratios gametes AB Ah aB ah 



produced 31 13 



by A Bah y i i 7 



in various 15 i i 15 



cases 31 I I 31 



etc. 



