The Role of the Chromosomes 



211 



and may be reared ad libitum on ripe bananas to which a 

 little yeast has been added. The fly has many variable char- 

 acters which are well marked and readily Mendelized. The 

 chromosomes also are well defined, so that it furnishes an 

 exceptionally good subject for studies of this character. 



The fruit fly has typically four pairs of chromosomes, and 

 thus far more than 400 distinct characters have been found. 

 Now if these characters has each its determiner in one of 

 the chromosomes, it is obvious that each chromosome must 



B 



DIAGRAMS ILLUSTRATING THE DISTRIBUTION OF THE SEX CHROMSOMES 



AT MATURATION 



A, in the female; B, in the male; C, the resulting possible combina- 

 tions in fertilization. A and B from Morgan, " Heredity and Sex," 

 by permision of the Columbia University Press; C from Loeb, after 

 Wilson. 



carry a large number of characters. That being so, all those 

 characters which are lodged in one chromosome should be 

 inherited as a unit. And that is precisely what happens in 

 many cases, giving rise to the phenomenon known as "link- 

 age." One of the best known examples of this is shown by 

 certain sex-linked (sometimes incorrectly called "sex-limited" 

 characters.) 



