NON-DISJUNCTION OF SEX CHROMOSOMES 589. 



by the no-X sperm will give a male (X — ), whose only X came 

 from his mother, and hence determines him as white. The one- 

 X eggs give, then, the normal criss-cross result characteristic 

 of sex linked inheritance, (white 9 X red d" = red 9 + white 

 d"). The two-X egg will give females only, since it is already 

 duplex with respect to the sex difTerentiator. (3) If this egg is 

 fertilized by a no-X sperm, there should result a female (X X), 

 both of whose sex chromosomes are maternal, and would thus 

 produce white. (4) The two-X egg, fertilized by the one-X 

 sperm, will produce an individual with three allelomorphic sex 

 chromosomes, two of which are white, and the other red bearing. 

 Thi.s fly will, if viable, be a red female, indistinguishable from 

 the large class above. (5) The no-X egg by the one-X sperm 

 will produce a male (X — ), differing from the normal male in 

 that his X is not maternal but paternal, and would produce a 

 red fly. (6) The no-X egg by the no-X sperm will produce a 

 zygote with no X, and such a zygote, I believe, is not viable. 

 , The presence of the white gene in the maternal chromosomes 

 gives us a means of disting-uishing between all the classes, except 

 between the three-X and the heteroxygous red X X types. 



Non-disjunction of the other chromosomes (autosomes) has not 

 been observed in these crosses. 



If 10 per cent of the eggs of such a female matured in a non- 

 disjunctional manner the proportions of the Fi would be: 



Females Males 



white 9 red 9 white cf red cf 



5% 95% (or 90%) 90% 5% 



The evidence in support of these conclusions is as follows. 

 From time to time in an Fi where sex-linked characters were 

 concerned, females or males have arisen which I was unable to 

 explain on an}* current Mendelian hypothesis. Some of these 

 I bred, and the offspring were as hard to explain as the original 

 exceptions. One of the first cases was the following. I crossed 

 a recessive pink female, of a stock which had been pure for some 

 twenty-five generations, to a male which was a triple recessive, 

 namely, white miniature black. The normal F, expectation for 



