THE GENETIC SYSTEM AS A WHOLE 167 



from the father into his daughters, all the genes, and all 

 the characteristics that they produce, pass together into the 

 daughters. Here of course some of the recessive character- 

 istics may remain hidden in the presence of dominant genes 

 in the second X-chromosome carried by the daughters. But 

 all may reappear anew when in a later generation this X- 

 chromosome is again by itself in a male. Except in so far as 

 there is exchange or crossing-over with the other chromo- 

 some of the pair, all the characteristics that depend on the 

 genes of a particular chromosome pass together into the 

 individual that receives that chromosome, and this continues 

 for generation after generation. 



Thus, in Drosophila, suppose that a male has white eyes 

 and yellow body, two characteristics connected with genes 

 in the single X-chromosome. He is mated with a female that 

 has red eyes and gray body, again owing to genes in the 

 X-chromosome. In the daughters of these two, the two types 

 of X-chromosomes are present together. These daughters 

 produce offspring in a third generation (see figure 32). 

 Some of their sons receive the X-chromosome that came 

 from the original male; these will have white eyes and yel- 

 low body. Other sons receive the X-chromosome that came 

 from the original female; these have red eyes and gray 

 body. Only a very minute proportion of the offspring have 

 the reverse combinations of characteristics — white eyes 

 with gray body, and red eyes with yellow body. The two 

 characters originally together — white and yellow — appear 

 to be linked, and the same is true of the other combination, 

 red and gray. The original combinations do not readily sep- 

 arate. Figure 32 shows a diagram of these relations. 



This linkage of characters in heredity was observed be- 

 fore it was known that characteristics depend on chromo- 

 somes. Many theories were invented to account for it, until 

 it was discovered that it is due to the fact that the linked 

 characters depend on genes that are in the same chromo- 



