34 GENETICS 



way that they produce bar-eyes. Others have become defec- 

 tive in such a way that they produce white eyes. 



These arc not the only kinds of changed X-chromosomes. 

 Many different sorts of peculiarities have been found that 

 thus follow the distribution of a particular type of X-chro- 

 mosome, being manifested in the body of the individual 

 when that type of X is the only type present. Up to 1925, 

 a hundred and thirty diverse types of X-chromosomes had 

 been detected scattered among the different individuals in 

 the fruit-fly — each type giving rise to some special peculiar- 

 ity. Many are known also in other organisms. In man a 

 considerable number are known, giving rise mainly to de- 

 fects or diseases. Among the characteristics dependent on 

 changed X-chromosomes in man are some types of color- 

 blindness, also a blood disease known as haemophilia. They 

 follow exactly the distribution of a particular type of 

 X-chromosome. This, of course, shows that in man too the 

 male has but a single X-chromosome, while the female has 

 two, for this is the only way in which that method of in- 

 heritance can be brought about. 



Thus it is established that when one X-chromosome is 

 substituted for another, this commonly makes a difference 

 in the characteristics of the individuals. Since there are 

 many types of X-chromosomes distributed among the indi- 

 viduals of a single species, such substitutions can be made 

 in many different ways, giving many different types of re- 

 sults. In the fruit-fly, some X-chromosomes produce red 

 eyes, others produce white eyes, others buff-colored, or 

 eosin-colored, or various other colored eyes. Some X-chro- 

 mosomes produce long wings, others short wings — and so 

 one could list a hundred or more diverse kinds of effects 

 of diverse X-chromosomes. Similar long lists could be made 

 for certain other organisms, and a considerable number 

 of diverse types of X-chromosomes are known even in man. 



The other chromosomes present somewhat more com- 



