CONSEQUENCES OF CHANGE 



is lost; the dicentric is stretched between the two poles and promptly 

 broken, unless, indeed, it is long enough to join the two daugliter 

 nuclei. The two broken cliromatids, even if the break is exactly 

 midway, are bound to be deficient in the genes contained in the 

 acentric fragment and the nuclei carrying them die — at least in a 

 diploid. The products of single crossing-over are therefore always 

 lost. 



Units of Recombination 



We must now consider what relation these changes in number 

 and structure of chromosomes have to the type of change ordinarily 

 studied in mendelian experiments and to its mode of inheritance. 



In triploids the unit of change, the whole nucleus, breaks up into 

 its elements, the separate chromosomes, in the next generation. With 

 single-chromosome and structural changes the unit of change may 

 persist as a unit of heredity: in heterozygotes the abnormality may 

 segregate from normality as a mendelian difference. But these 

 changes have certain characteristics of their own by means of which 

 they may be distinguished from the ordinary mendelian factor. In 

 the homozygote, inversions and interchanges affect the linkages of 

 blocks of genes in the ways we should expect; inversions, by setting 

 the genes of a linkage group into a new order (see Fig. 31); inter- 

 changes, by separating genes previously linked and bringing together 

 genes previously in separate linkage groups. In the heterozygote, 

 however, their effects are peculiar. 



In inversion heterozygotes, single crossing-over within the 

 inversion produces non-viable deficiencies. Only when double 

 crossing-over occurs and chances to be reciprocal, i.e. between the 

 same pair of chromatids and therefore genetically compensating, 

 can there be recombination of genes within an inversion. Short 

 inversions therefore suppress recombination within them. They 

 establish a block of genes as a new single unit of recombination 

 in all descendants which are heterozygous for the old and new 

 arrangements. They work on the principle already suggested in 

 our consideration of the agouti gene in mice. 



In the light of these observations we find inversions appearing 

 as "cross-over suppressors" when heterozygous in breeding experi- 

 ments. Sturtevant found such "genes" in Drosophila which he 



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