NON-DISJUNCTION OF SEX CHROMOSOMES 601 



This is an extremely close approximation to the result expected 

 on the assumption that the grandfather was vermilion and pink 

 as well as white. Thus, although to sight the vermilion was 

 masked by the white, its reappearance in F2 makes another in- 

 stance where the whole set of sex-linked factors of the father 

 appeared in his exceptional sons in their original combination 

 exactly. The simplest hypothesis is one which assumes a common 

 carrier for all sex-linked factors and the sex differentiator. Just 

 such a carrier is offered by the sex chromosome. 



Dexter ('12, p. 190), crossed a yellow female heterozygous for 

 white to a gray white male. There were produced: 



gray red 9 gray white 9 yellow redcf yellow whitecf gray wh.c?" 



116 116 *82 100 2 



These gray white males which Dexter was unable to account 

 for in any way, are exactly like their father, as is characteristic 

 of non-disjunctional sons. 



WHien the white eyed mutant first appeared, it was mated 

 to normal red females. In Fi there appeared five white males, 

 which Morgan explained as further mutations. It seems to me 

 more likely that these are non-disjunctional males. 



