GENETIC SYSTEM: RELATION TO CHARACTERISTICS 8^ 



As we shall see in detail, many different characteristics 

 are known that are thus due to defects or alterations in 

 certain X-chromosomes. Such characteristics show very re- 

 markable rules of inheritance. A number of such character- 

 istics were known in man before their relation to chromo- 

 somes were discovered; their method of inheritance was 

 extremely puzzling. It will be worth while to observe the 

 rules of inheritance of such characters, as they appear when 

 the relation to X-chromosomes is not brought out. In man 

 there has long been known a defect called haemophilia, 

 which follows this method of inheritance. It is due to a 

 defective X-chromosome, and shows itself in the fact that 

 the blood does not coagulate on exposure to the air, so that 

 if wounded the defective individuals are likely to bleed to 

 death. The defective X-chromosome fails to produce cer- 

 tain materials that are required for coagulation of the 

 blood. But if a normal X is present in addition to the de- 

 fective one, the necessary material is produced, and the 

 blood coagulates normally. Haemophilia is thus a recessive 

 character. Color-blindness is another recessive character- 

 istic that is due to a defective X-chromosome. 



The following are the rules of inheritance shown by such 

 a recessive character resulting from a defective X- 

 chromosome (see the diagram, figure 24). 



(a) The individual affected by the abnormality is usu- 

 ally a male. 



(b) When such an affected male mates with a normal 

 female, none of the children are affected. It appears as if 

 the defect were not inherited. 



(c) But in the grandchildren the defect reappears; it 

 has "skipped a generation." 



(d) But not all the grandchildren have the defect. 

 There is a peculiar distribution of the defect among them, 

 as follows: 



(i) None of the sons' children have the defect; nor 



