138 GENETICS 



more than two diverse characteristics are found thus to fol- 

 low one single X-chromosome. Three, four, five or six have 

 been found following one X-chromosome. And yet, in other 

 cases, each of these same characteristics is connected with 

 a separate X-chromosome and follows it in its distribution. 

 That is, two or more characteristics may either be con- 

 nected with separate X-chromosomes, or they may be all 

 connected with the same X-chromosome. It is to be under- 

 stood that these are experimental facts, demonstrated in 

 thousands upon thousands of breeding experiments. 



In view of these facts, an important question arises. 

 When several characteristics follow a single chromosome, 

 are they following different parts of it? Has the X-chromo- 

 some different parts that can be modified separately, as was 

 suggested on other grounds on a former page? Or is the 

 chromosome an inseparable unit, so constituted however as 

 to cause in the individual that bears it several different pe- 

 culiarities? 



Experimental breeding answers this question clearly. It 

 is most important to observe what this answer is and how 

 it is obtained, for it constitutes one of the foundation facts 

 of heredity. The answer turns out to be, as we shall see, 

 that the chromosome is indeed composed of different parts, 

 which have different effects in development. These different 

 parts, as we shall see, can be separated, each carrying with 

 it its own peculiar effect. The diverse separable parts of the 

 chromosome having diverse effects are what are called 

 genes. Their existence is solidly demonstrated, and can be 

 verified by anyone who will take the necessary trouble. 



The type of experiment that proves this to be true is as 

 follows: Select in the fruit-fly a male that has white eyes 

 and yellow body. These characteristics are both connected 

 with the single X-chromosome carried by the male, as can 

 be proved by such breeding experiments as have been de- 

 scribed above. Now mate such a male with a female whose 



