SEX 



93 



and the 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, F t 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 lacti- 

 color were females. Now this seemingly complex 

 collection of facts is readily explained if we make 

 the following three assumptions : 



(1) The grossulariata character () is dominant 

 to the lacticolor character (g]. This is obviously 

 justified by the experiments, for, leaving the sex 

 distribution out of account, we get the expected 

 3 : i ratio from F X x F x , 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 con- 

 stitution of a female is consequently Ff, and of a 

 male ff. This assumption 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 zy gates'- (FfGg) must always be 

 females, and on this assumption will produce gametes 

 Fg and fG in equal numbers. 



We may now construct a scheme for com- 

 parison with that on page 92 to show how these 

 assumptions explain the experimental results. The 



