x SEX 101 



four sorts were produced in approximately equal numbers. 

 In such a family there was no special association of either 

 of the two colour varieties with one sex rather than the 

 other. But the reverse cross, Fi female by lacticolor 

 male, gave a very different result. As in the previous 

 cross, such families contained equal numbers of the 

 normal form and of the recessive variety. But all of the 

 normal grossulariata were males, while all the lacticolor 

 were females. Now this seemingly complex collection 

 of facts is readily explained if we make the following 

 three assumptions : — 



(i) The grossulariata character (G) is dominant to the 

 lacticolor character (g) . This is obviously justified by the 

 experiments, for, leaving the sex distribution out of ac- 

 count, we get the expected 3 : 1 ratio from Fi X Fi, and 

 also the expected ratio of equality when the heterozygote 

 is crossed with the recessive. 



(2) The female is heterozygous for a dominant factor 

 (F) which is lacking in the male. The constitution of a 

 female is consequently Ff, and of a male ff. This as- 

 sumption is in harmony with the fact that the sexes are 

 produced in approximately equal numbers. 



(3) There exists repulsion between the factors G and F 

 in a zygote which is heterozygous for them both. Such 

 zygotes (FfGg) must always be females, and on this 

 assumption will produce gametes Fg and JG in equal 

 numbers. 



