THE EFFECT OF LINKAGE 



phcnotype oiily in so far as there is free phenotypic variability, 

 which becomes fixed as a result of the selection. Since the method 

 of selection was constant throughout Sismanidis' experiment, we 

 must suppose that free variability was low or absent during the 

 periods of little advance, but that it was present when the two chief 

 responses to selection occurred. That one of these came after genera- 

 tion 14 shows that variability was present during the preceding 

 generations; but since the response was delayed, the variability must 

 have been largely in the potential state. 



Now potential variability is freed by segregation and recombina- 

 tion. In the present case the chromosome tests showed that the 

 changes occurred within whole chromosomes. In other words, the 

 effective recombination must have been of linked genes, as a result 

 of crossing-over between them. Originally in the form of balanced 

 linked combinations of polygenes, crossing-over must have led to 

 the production of less well balanced genotypes, giving more extreme 

 pheno types, i.e. to the freeing of variability which must have 

 been present in the group of parents with which the experiment 

 began. 



Linkage must have the general effect of tending to maintain 

 genes in the same combinations even in cross-bred and heterozygous 

 populations. If we cross AABB and aahh, where the genes are linked, 

 the combinations aB and Ah wiU be produced in the segregating 

 generation only so far as crossing-over gives rise to recombination. 

 If the linkage is close, they wiU be uncommon for some time. 

 The combinations AB and ab will be released equally slowly where 

 the parents are AAhb and aaBB. Thus, while the proportion of 

 heterozygotes, and hence of heterozygotic potential variability, wiU 

 depend solely on the breeding system, the rate of flow of free 

 variabiUty to the homozygotic potential state and back wiU be 

 governed by linkage as well. The flow will be at its maximum when 

 recombination is free, for a double heterozygote then produces 

 equal numbers of AB, ah, Ah and aB combinations no matter what 

 its origin may have been. But at the other extreme, with close 

 linkage, the interchange of variability between the free and 

 homozygotic potential states wiU be small: after passing into the 

 heterozygotic pool, free variability wiU mostly emerge as free 

 variability, and homozygotic potential as homozygotic potential. 



287 



