SEX-CHROMOSOMES AND INHERITANOE 199 



As many as 20 different lethals have been found in 

 the Z-chromosomes of Drosophila. Their occurrence in 

 these chromosomes is first noticed by the appearance of 

 such exceptional sex ratios. Lethal factors like these 

 need not be thought of as different in kind from any 

 other mutant factors. They may mean only that the 

 changes that they cause are of such a kind, structural 

 or physiological, that the affected individual cannot 

 develop normally. Some of the lethals may be fatal in 



Fig. 91. — Scheme showing the transmission of a lethal sex-linked factor in an X-chromoiome 



the black one in the diagram. 



the egg stages, others are known to cause the death of 

 the larvae, others probably act on the pupae, and a few- 

 even allow an affected male to occasionally come through. 

 In man and in several other mammals there is at birth 

 a slight excess of males over females. Since male babies 

 die oftener than females, the difference has been said to be 

 an '^adaptation,'' with the implication that it calls for no 

 further explanation. Several possible solutions suggest 

 themselves. The male-producing sperm bearing the sex- 

 chromosome may more frequently develop abnormally 

 than the female-producing sperm. Again, since the sper- 

 matozoa must, by their own activity, travel the entire 



