90 GENETICS 



in males, are commonly called sex-linked characteristics. 

 In the case of certain serious defects, these rules are 

 modified by the fact that individuals with two defective X's 

 (females) cannot live and develop. Haemophilia for ex- 

 ample is not certainly known to occur in females. Any fe- 

 male that gets two X's defective in this way simply does not 

 live. Color-blindness however illustrates well all the rules 

 set forth above, and the same is true for many such de- 

 fects in animals. 



Abnormal Distribution of X-Chromosomes 



What will happen if the X-chromosomes by accident be- 

 come irregularly distributed? We have seen in Chapter III 

 that sometimes the X-chromosomes are indeed irregularly 

 distributed. Will the sex-linked characteristics continue to 

 follow them; will the defective characteristics show the 

 same irregular distribution as do the X's? 



Such cases in great number have been fully observed. It 

 Is found that the sex-linked characteristics do indeed follow 

 the X-chromosomes wherever they go. This proves con- 

 clusively (if there were any possible doubt in view of the 

 extraordinary course normally followed by such charac- 

 ters) that it is indeed the X-chromosomes on which the 

 characters depend. The matter is one of importance and in- 

 terest, so that it will be worth while to examine carefully 

 certain typical cases of the result of irregular distribution 

 of X-chromosomes. 



Normal Distributions (figure 26, A) : Sometimes the 

 body of Drosophila is yellow instead of the normal gray; 

 this is a recessive character due to a defect in the X- 

 chromosomes. Suppose that we mate together a female that 

 has a yellow body and a male that has the normal gray 

 color. The female has two modified X-chromosomes, which 

 we may represent by XX. The male has its X unmodified; 



