GENETIC SYSTEM: RELATION TO CHARACTERISTICS 87 



(i) Only males have the defect. 



(2) It skips a generation, and reappears only in part of 

 the grandsons through the daughters. 



(3) It is thus inherited through normal females. 



(4) But it is never inherited through normal males. 

 The above is the usual course of inheritance; it holds for 



the great majority of cases. 



(f) But observers were perplexed by finding that in very 

 rare cases all these rules are broken. The following were 

 then observed (see figure 25) : 



(i) The individual affected is a male, as before. 



(2) He marries a normal female; and now some of the 

 children are affected. 



(3) But not all the children are affected; on the average 

 about one-half of all. 



(4) Those affected include both males and females, in 

 equal number. Thus here we find that the rule that only 

 males are affected does not hold; females also may have 

 the defect. 



(g) When now one of these rare defective females is 

 mated with a normal male: 



(i) All the sons produced are affected. 



(2) None of the daughters are affected. 



Thus "the sons inherit from their mothers, the daugh- 

 ters from their fathers," in such a case. 



One further important fact was observed: 



(h) If the mother and father are both affected, then all 

 children are affected, including daughters as well as sons. 



Imagine trying to get intelligible rules of inheritance 

 from such a set of seemingly contradictory observations ! 

 Even the rules commonly followed seem arbitrary and in- 

 comprehensible. And at times they are all broken; some- 

 times one result is produced, sometimes another. 



But when it was discovered that such defects are due to a 

 defective X-chromosome, and that they appear wherever 



