SELECTION AND VARIABILITY 



Where natural selection is penalizing the less well balanced 

 combinations of genes in a chromosome, genes of opposing or 

 balancing effects must tend to become tied together by the linkage. 

 The population will then show a smaller spread round the optimum 

 plienotypc than it would in the absence ot linkage. Fitness will 

 consequently be higher, as it is with an inbreeding system. But the 

 more extreme combinations will still be produced on occasion by 

 crossing-over ; and, even though less common, they will still afford 

 the material for selective adjustment of the genotype to changing 

 environment. In fact they will be just as effective in this way as if 

 they were released with the unlinked frequencies. Flexibility will 

 therefore be fully maintained at the same time that fitness is 

 increased. In a large measure, linkage can reconcile their rival 

 needs. 



Tlie Control of Recombination 



We may now ask ourselves: how tight is linkage in practice? 

 How much crossing-over actually takes place between paired 

 chromosomes e The chiasmata have shown us that all chromosomes 

 in all species of sexually reproducing organism undergo crossing- 

 over, and that the position of crossing-over in any particular pair 

 varies from cell to cell and therefore, obviously, from generation 

 to generation. It sometimes happens, for example in Viciafaha and 

 in many species of lilies, tiiat each pair of chromosomes forms an 

 average of five or more chiasmata — far more than the bare minimum 

 number needed to hold them together at the first metaphase of 

 meiosis. But this is rare. In general, what we find is a close restriction 

 of the number of chiasmata to this mechanical need. Some variation 

 in number is doubtless required if variation in position is to be 

 maintained, and two-armed chromosome pairs frequently form two 

 chiasmata. 



The adjustment of chiasmata to a minimum has been neatly 

 shown in maize. An "asynaptic" gene, in the homozygous state, 

 reduces the frequency of chiasmata at meiosis. Indeed in extreme 

 cells of extreme plants it causes the chromosomes to fall apart at 

 the end of pachytene without any formation of chiasmata at all, so 

 that they are all unpaired at diakinesis and metaphase. Typical 

 samples are shown in Table 25. 



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