CORRELATED RESPONSE 



price of linkage is therefore that, apart from the primary character, 

 the immediate overall response to selection is poorer. Time is 



At Bi A2 ba 



Ol bl 02 B2 



Az b; 



Ai bl 02 bz 



01 Bl A2 B2 



Ai bl A2 bz 



Ol Bl 02 B2 



Fig. 75. — The mechanism of correlated response to selection. The two A-a genes 

 represent members of one polygenic system and the two B-b genes of another all 

 carried on the same chromosome. 



Both systems are balanced in the sense that each of the original chromosomes 

 carry genes of opposing tendencies, as indicated by capital and small letters. Selection 

 will pick out the chromosomes which have lost this balance by recombination 

 bringing together two genes of reinforcing tendency either in the direction indicated 

 by capitals or that by small letters. According to the position of the crossing-over, 

 upon which recombination depends, (i) the A system may be unbalanced, the B sys- 

 tem remains balanced (left); (ii) the B system may be unbalanced, the A system 

 remaining balanced (right), or (iii) both systems may be unbalanced (centre). Then 

 in (i) selection c^n be effectively practised for the character controlled by the A sys- 

 tem without that controlled by B changing, or (ii) selection can be effective for the 

 B character without the A character changing. But in (iii) effective selection for either 

 character must also result in change in the other, solely by virtue of the linkage of the 

 genes. This is correlated response, and it may be in either direction for the subor- 

 dinate character, again according to the precise linkage arrangement. 



luccded for recombination to permit the attainment of a full new 

 adjustment in the primary character without deleterious effects on 



299 



