SEX-CHROMOSOMES AND INHERITANCE 203 



mothers by red-eyed fathers have normally only red-eyed 

 daughters. These exceptional white-eyed females (XXY) 

 must repeat the phenomena of non-disjunction, and it has 

 been found that they do so invariably. The white-eyed 

 male XY is normal ; the other male should produce some 

 XY sperm and thus transmit both X and Y to some of his 

 daughters. Such daughters as get both X and Y from 

 the entering sperm should show non-disjunction. This 

 has been proven to occur. 



An analysis of the data has shown that two of the four 

 types of eggs are more common than the other two. As 

 indicated in both diagrams the types of eggs that result 

 after X and X have united occurs in 92 per cent, of the 

 cases, and since in this type the unmated Y has a random 

 distribution, the XY egg is found in 46 per cent, of cases 

 and the X egg in 46 per cent. The more uncommon type 

 of egg would be expected to result if X and Y united and 

 then separated while the other X had a random distribu- 

 tion.^ Eight per cent, of such cases occur, giving XX eggs 

 in 4 per cent., and Y eggs in the other 4 per cent, of cases. 



These results not only furnish very strong proof of 

 the chromosome theory of sex, but serve also to show how 

 a knowledge of the actual mechanism involved leads to 

 the discovery of how a change in the mechanism gives a 

 new output. The conclusion that females behaving in 

 this way must contain a Z-chromosome was confirmed 

 by the cytological demonstration that showed in them two 

 X^s and a Y, 



^ Since this was written it has been found that after XY synapsis the free 

 X always goes to the same pole as the synapsed X. 



