THE GENETIC SYSTEM AS A WHOLE 1 69 



children (seen particularly in the grandsons). In addition 

 there are a very few "cross-overs," having the new combi- 

 nations white-gray in some individuals, red-yellow in others. 

 But almost all the grandchildren get both the characteris- 

 tics from the same grandparent. If the original grand- 

 parents have the other combinations, gray-white and yellow- 

 red, then the majority of the grandchildren show these 

 combinations. 



Thus just what combination of characteristics the indi- 

 vidual grandchildren have depends on what characteristics 

 were united in the grandparents. But this is the case of 

 course only for characteristics whose genes lie in the same 

 chromosome. If two characteristics depend on genes that are 

 in different pairs of chromosomes, then the combinations 

 that were present in the grandparents are no more frequent 

 in the descendants than are the reverse combinations. The 

 combinations in such a case are those indicated in table 2 on 

 page 117. 



It is in many respects most convenient to think of linkage 

 in terms of the gametes. The combination of genes that are 

 present in the gametes can often be determined by examina- 

 tion of the offspring produced. With relation to the gametes, 

 linkage can be characterized as follows: When an individual 

 (zygote) is formed by the union of two gametes, each bear- 

 ing in one of its chromosomes a certain combination of 

 linked genes, this same individual later produces gametes of 

 which the majority have these same combinations of linked 

 genes. Often also there is a minority of gametes that have 

 the genes combined in a new way, as a result of crossing- 

 over. These relations are shown in figure 39. 



In the fruit-fly, crossing-over occurs only in the females, 

 not in the males. Thus if a male individual is formed by the 

 union of two gametes carrying certain combinations of 

 linked genes, then later such a male produces only gametes 

 having the same combinations of linked genes. This fact is 



