158 



CHAPTER 19 



Structural changes may result also after 

 three breaks. When all three occur in one 

 chromosome, the two interstitial pieces may 

 exchange positions, producing what is called 

 a shift. Two breaks in one chromosome and 

 one in a nonhomolog can result in the inter- 

 stitial piece of the first chromosome being 

 inserted into the second. This result is called 

 transposition. 



In this and in the preceding Chapter, it 

 has been pointed out that a change in ploidy 

 may survive in nature when it either causes 

 no shift in chromosome balance (because it 

 deals with whole genomes), or involves 

 eucentric aneuploidy in which small segments 

 of chromosomes are hypo- or hyperploid. 

 For, in the latter case, the deficient or dupli- 

 cated genes are limited in number and produce 

 only moderate phenotypic effects. On the 

 reasonable assumption that the greater the 

 amount of chromosomal material, the greater 

 the complexity possible in an organism, and 

 correspondingly, the greater the possible 

 diversity in its phenotype and adaptiveness, 

 viable changes in ploidy are particularly im- 

 portant in organic evolution. In view of this 

 it becomes desirable to consider the different 

 ways in which small numbers of genes may be 

 added to a genome following breakage. 



Two methods of increasing gene number 

 following breakage have already been de- 

 scribed. One of these dealt with two breaks 

 in the same chromosome. After breakage, 

 the entire chromosome replicated, after which 

 the broken ends joined so as to form a chro- 

 mosome with the interstitial piece duplicated 

 (cf. p. 156). The other method involved a 

 pair of homologs each having one break in a 

 different region, followed by eucentric cross 

 union (cf. p. 156). A third mechanism in- 

 volves the heterozygote for a shift. If, in this 

 case, the homologs pair in the region of the 

 shift and a chiasma forms within it, you will 

 see, by tracing the resultant strands, that one 

 of the crossovers has a section in duplicate. 



FIGURE 19-6. Inversion heter- 

 ozygotes in corn {pacliynema) 

 {courtesy of D. T. Morgan, Jr.) 

 and in Drosophila {salivary 

 gland) {courtesy of M. De- 

 merec). 



