THE BREAKDOWN OF CONTINUITY 



uniformity which inbreeding can give without its depression of 

 vigour and fertiUty. 



Interchange and polyploidy avoid gross recombination between 

 chromosomes, and it may be, therefore, that their chief advantage 

 lies in the short cut each offers to inbreeding. Inversions, on the 

 other hand, limit recombination within chromosomes. They can 

 tie together, or peg, the very combinations of genes which will 

 have been built up by the normal process of linkage in the way 

 we have discussed in the previous chapter. It is also known that 

 inversion heterozygosity need not lead to any marked sterility on 

 the female side, for the inviable cross-over chromatids may be 

 confmed to those products of meiosis (spores or polar bodies) from 

 which the egg never develops. 



Floating and Fixed Discontinuity 



Inversion, interchange and polyploidy have one property in 

 common. They abolish the recombination of genes which would 

 otherwise recombine more or less freely, and in so doing they give 

 rise to super-genes, units of transmission greater than the gene 

 itself They peg the combinations of genes which have been successful 

 in leading to high adaptation in the past. This pegging is not, 

 however, a fmal process since the avoidance of recombination is 

 seldom quite complete. In polyploids, aberrant pairing occasionally 

 leads to aberrant recombination, while rare double crossing-over 

 in interchange and inversion heterozygotes can lead to some 

 redistribution of the constituents of the super-gene and so to the 

 origin of spurious mutations. The result is that some genie 

 heterogeneity may occur within one structural type. And even 

 without this recombination a similar result would come about in 

 time by virtue of true mutation alone. 



The consequences of this development have been followed by 

 Dobzhansky in DrosopJiila psendoohscura. In this fly the third 

 chromosome (to a much greater extent than its fellows) exists in a 

 number of sequences derived from one another by inversions as 

 revealed by polytene chromosomes. These inversions overlap one 

 another so that they cannot recombine as units, and it has 

 consequently been possible to work out the phylogenetic tree of 



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