I02 GENETICS 



if the defective X is the only kind that is present. It is well 

 known that, in general, the death rate is higher in males 

 than in females; this is probably due to the fact that males 

 have but one X-chromosome. 



8. Among the different individuals of a species are scat- 

 tered a great number of different types of X-chromosomes 

 having different effects on development. Many of the di- 

 verse X-chromosomes are distinctly defective, causing per- 

 sonal defects in the individuals that bear them. Others are 

 not defective. 



9. Defects or modifications in X-chromosomes affect in 

 different cases all parts and functions of the organism. 



The Role of the X-Chromosomes in Development 



What do the facts brought out in the preceding sec- 

 tions show as to the physiological functions of the X- 

 chromosomes; as to their role in development? 



For every kind of effect resulting from defects or modifi- 

 cations in the X-chromosomes, there is a corresponding 

 (opposed or diverse) action of normal or unmodified X- 

 chromosomes. Since defective X-chromosomes of a certain 

 type produce in Drosophila short, ill-developed wings, it 

 follows that the normal X-chromosomes act on the develop- 

 ment in such a way as to give long, well-developed wings. 

 For if we substitute the normal X for the defective one, 

 this causes the normal well-developed wings to be produced. 

 Similar reasoning applies to all the defective conditions 

 that result from defective X's; the normal X's act in such 

 a way as to produce the corresponding normal conditions. 



It follows therefore that the normal X-chromosomes 

 play a role in the production of eye color and structure, 

 body color and structure, in the development of the legs, 

 the wings, the bristles, in producing normal health, re- 

 sistance and vigor, and in various physiological processes. 



