224 



HEREDITY AND SEX 



the expectation. Thus by utiHzing sex chromosomes 

 that carry other sex-Unked characters than white eyes, 

 it can be shown that the results are really due to the 

 whole sex chromosome being involved, and not to 

 parts of it. The result is of unusual interest in another 

 direction ; for it shows that the female-producing 



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V/ 



\W/ 



X 



X) 



x>; 





R.<J 9 



/frr/ 



^^\^ 



,9-5 



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P 



-it/,,/^ O 



ft/,,^0 



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90 



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Fig. 107. — Non-disjunction. A non-disjunction female produces four 

 types of eggs, viz., XY — X — XX — Y . Such a female will give, with a 

 normal male, XF, the classes indicated on the diagram. Y should be sub- 

 stituted for in the diagram. The non-disjunction 9 is XXF. 



sperm will make a male if it enters an egg from which 

 both sex chromosomes have been removed. It is 

 therefore not the female-producing sperm, as such, 

 that gives a female under normal conditions, but this 

 sperm plus the sex chromosome already present in 

 the egg that gives an additive result — a female. 



(4) In the group of nematode worms belonging es- 

 pecially to the genus Rhabditis, there are some extraor- 



