52 



INTROGRESSIVE HYBRIDIZATION 



a population of 144. It will be seen that the population 

 would be made up largely of the recombinations along a di- 

 agonal spindle through the figure (the '^recombination 

 spindle" of Chapter 3). Nothing like the recombinations 

 of the upper left-hand corner or the lower right-hand comer 

 could appear. In other words, pubescence would tend 







1 



Fig. 11. Frequencies from the 

 same cross as Fig. 10 if the genes 

 A, B, C, and D were linked and 

 in that order and if there were 

 regularly one crossover, but no 

 more. Expectations in an F2 of 

 144 plants if crossovers were 

 equally frequent at any point. 



12 3 4 



Fig. 12. The effect of localized 

 chiasmata upon the frequencies of 

 Fig. 11. Expectations in an F2 of 

 576 plants if crossing over be- 

 tween B and C were four times 

 as likely as between A and B or 

 C and D. 



strongly to be correlated with leaf shape ; hairy-obovate and 

 glabrous-ovate types would be common, in addition to in- 

 termediates like the Fi. Approaches to the extreme recom- 

 binations would be in the minority. Take, for instance, the 

 types of leaves which are intermediate between the Fi (2/2) 

 and the two extreme recombination corners 5/0 and 0/5. 

 They fall at 4/2 and 2/4 on Fig. 9. The first is a fairly obo- 

 vate leaf with scattered pubescence, the latter a distinctly 

 ovate leaf with quite heavy pubescence. Though theoret- 

 ically, individuals of these two types could occur, either of 

 them would be expected only once in 72 times, whereas 



