98 GENETICS 



JVhat Kinds of Characteristics Depend upon Alterations of 

 the X-Chromosomesf The Role of the X-Chromosomes 



in Development 



A great many characteristics, of various kinds, have been 

 found to be the result of modifications of particular X- 

 chromosomes, and thus to follow in later generations the 

 distribution of the descendants of those chromosomes. Ex- 

 amination of a number of these is desirable, both for their 

 own importance, and for the light they throw on the func- 

 tions of the X-chromosomes. 



In man,^ the following characteristics, among others, 

 dependent on diverse types of X-chromosomes, are known 

 from the fact that they show typical sex-linked inheritance. 



Haemophilia: Lack of coagulability in the blood. This 

 results from a serious defect in certain X-chromosomes. 

 The existence of defective X-chromosomes having this re- 

 sult shows that the normal X-chromosomes play a part in 

 supplying something necessary for producing normal blood 

 that coagulates properly. 



Color Blindness. The fact that defectiveness in X- 

 chromosomes causes color blindness shows that the normal 

 X's play a role in producing the normally functioning eyes. 



Night Blindness: Inability to see in a poor light. 



Nearsightedness of certain types. 



Progressive atrophy of the muscles (Gower's disease). 

 The fact that defects in X-chromosomes have this effect 

 shows that the normal X's play a role in the normal func- 

 tioning of the nerves and muscles. 



A considerable number of other sex-linked characters are 

 known in man. But man is a very unfavorable organism for 

 the study of inheritance. Yet even the little that is known 

 of sex-linked inheritance in man shows that the X- 

 chromosome plays a role in many diverse bodily functions. 



