Structural Changes in Chromosomes 



173 



produces one nucleus without the transloca- 

 tion and the other with the full transloca- 

 tion. Since euploid gametes are usually 

 formed, such translocation heterozygotes are 

 not at an appreciable reproductive disad- 

 vantage. 



Increasing Gene Number 



Both here and in Chapter 1 I , it has been 

 pointed out that a change in ploidy can sur- 

 vive in nature when it involves either no 

 shift in chromosome balance (because it 

 deals with whole genomes) or eucentric 

 aneuploidy due to small segments of chro- 

 mosomes which are hypo- or hyperploid. 

 In the latter cases, the number of deficient 

 or duplicated genes is small enough to pro- 

 duce a tolerable phenotypic effect. It is 

 reasonable to assume that the greater the 

 amount of chromosomal material, the greater 

 the complexity possible in an organism and. 

 consequently, the greater the diversity pos- 

 sible in its phenotype and adaptiveness. Ac- 

 cordingly, viable changes in ploidy must be 

 particularly important in organic evolution. 

 It is desirable, therefore, to specify some of 

 the different ways that small numbers of 

 genes can be added to a genome after break- 

 age. 



Two methods of increasing gene number 

 after breakage have already been described. 

 One requires two breaks in the same chro- 

 mosome; the entire chromosome then repli- 

 cates, after which the broken ends join to 

 form a chromosome with the interstitial 

 piece duplicated (p. 170); the other involves 

 each member of a pair of homologs break- 

 ing once in a different region before eucen- 

 tric cross union (p. 172). 



A third mechanism involves three breaks 

 in one chromosome. The two interstitial 

 pieces exchange positions, producing what 



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figure 12-7. Inversion het- 

 erozygotes in corn (pachy- 

 nema) (courtesy of D. T. 

 Morgan, Jr.) and in Drosoph- 

 ila (salivary gland) (courtesy 

 of M. D enter ec). 



