28 INTROGRESSIVE HYBRIDIZATION 



To find out the effect of linkage in a cross between two 

 species differing by a large number of genes, let us first con- 

 sider a hypothetical limiting case. Suppose the two species 

 differ by 50 genes and that these gene differences are more 

 or less uniformly distributed through 10 pairs of chromo- 

 somes. If there were no recombination within any of the 



50 independent genes 



Fig. 1. F2 frequency curves for a character controlled by 50 genes all 

 equal in effect, with and without linkage. 



chromosomes (and though such a case is certainly extreme 

 it is not unknown experimentally), each of the chromo- 

 somes would behave like a giant gene. Its 5 genes would 

 always segregate simultaneously. The segregation of 50 

 genes each on a separate chromosome would follow the curve 

 (1 : 2 : 1)^°. Their segregation if they were in 10 chromo- 

 somes with no crossing over would be represented by 

 (1:2: ly^. If in the first case we give each segregating 

 gene pair a value of 1 unit in determining the difference in 

 the character in question, then in the second example each of 



