XII THE CHROMOSOME THEORY 145 



mating the F^ females with black vestigial males the 

 resulting generation consists, as before, of the four 

 types, grey normals, grey vcstigials, black normals, 

 and black vestigials. But it is now the grey 

 vestigials and the black normals which outnumber 

 the other two classes by about 5 to i. These arc 

 of course the two original parental types, and the 

 explanation of their appearance in excess of the two 

 non-parental types lies, as before, in the excess of 

 the non - crossover gametes produced as compared 



^Black j^ Q Grey 

 ^normal ^ vestigial 



I I 



Black Q X ^ ^'^^y 9 ^'■^y X r^ Black 



stigial ^ ^ normal ^ normal ■ ^ vestigial 



vestig 



Grey Black Grey Grey Black Black 



vestigial normal normal vestigial normal vestigial 



50% 50% 8-5% 41-5% 41-5% 8-5% 



Fig. 43. 



For explanation see text. 



with crossover ones. In the case of the F^ male 

 too the result is strictly comparable with that 

 noticed in the preceding cross. When mated with 

 the double recessive, the black vestigial female, he 

 produces only grey vestigial and black normal off- 

 spring in approximately equal numbers. It must 

 be inferred that the male produces only the two 

 parental types of chromosome, and that no crossing 

 over occurs in this sex. This is true, not only of 

 the particular case under discussion, but of all cases 

 where the constitution of the male has been investi- 

 gated by this method. Why this crossing over 



L 



