86 



GENETICS 



does it reappear In any of their descendants. So far as the 

 descendants through the sons are concerned, the defect has 

 disappeared. 



(2) But some of the daughters' children have the de- 

 fect. 



(3) None of the daughters' daughters have the defect. 



Mother 7athep 



Father defeotlre 

 Mother not defeotiv* 



Wone 

 defeetlT* 



Balf the sons defective 



Mo offspring defeotlv* 



Figure 24. Diagram of the descent of a recessive defective X- 

 chromosome, derived from the original male parent, in cases where 

 there is no mating among relatives. Defective X represented in out- 

 line, normal X in solid black. The sons and daughters in Fi, de- 

 rived from the original mating, are conceived to mate with the un- 

 related individuals A and B, whose chromosomes are shown as 

 stippled — the X-chromosomes of A and B not being defective. De- 

 fective individuals are marked d. In Fi, none of the offspring are 

 defective. In F2, half the daughters' sons are defective, while none 

 of the sons' offspring (nor later descendants) are defective, since they 

 lack entirely the defective X. 



(4) But some of the daughters' sons have It. On the 

 average It turns out that about one-half of the daughters' 

 sons have the defect. 



(e) In later generations It continues as a rule in the 

 way we have described, that is: 



