L68 



CHAPTER 12 



in a dicentric rod or ring, respectively, as 

 can be seen by drawing the appropriate con- 

 figurations. 



Of course, a nondividing nucleus, in which 

 breakage or another structural change oc- 

 curs, is still euploid. The first occurrence of 

 hypoploid)/ or hyperploidy (aneuploidy due 

 to an excess of genes or chromosome parts) 

 is in the daughter nuclei formed by such a 

 nucleus. This delay in producing an aneu- 

 ploid nucleus should be remembered when 

 we state that chromosomes with small de- 

 ficiencies can be lethal when homozygous, 

 and detrimental when heterozygous; chro- 

 mosomes with large deficiencies usually act 

 as dominant lethals in the next cell genera- 

 tion. Remember also that we have ignored 

 — and shall continue to do so for the rest of 

 the chapter — the usual consequence of two 

 breaks, that is. restitution for all ends pro- 

 duced by breakage. 



2. Inversion 



Another structural consequence of two 

 breaks in the same chromosome is repre- 

 sented in Figure 1 2-2 b and d. In this case, 

 the middle piece is inverted with respect to 

 the end pieces and undergoes exchange 

 unions with them. The result which is either 

 a paracentric or a pericentric inversion (Fig- 

 ure 12-2 b and d), is due to the middle seg- 

 ment moving while the ends are relatively 

 stationary, or the reverse. Note that inver- 

 sion is a euploid rearrangement. 



Structural rearrangements in chromo- 

 somes can occur in either the somatic or 

 the germ line. An inversion which occurs 

 in the germ line may be retained in the pop- 

 ulation long enough to become homozygous 

 in some individuals. Meiotic behavior is 

 normal in such inversion homozygotes 

 whether or not the tetrad undergoes cross- 

 ing over, since all the strands in the tetrad 

 are identically inverted. Other individuals 

 in the population, however, may possess one 

 inverted and one noninverted homolog, be- 



ing inversion heterozygotes. Provided the 

 inversion is wry small, these homologs will 

 pair properly everywhere but in the inverted 

 region. Because the homologs cannot twist 

 enough to make homologous loci meet in so 

 short a region, they will fail to synapse and 

 no crossing over will occur. Insofar as 

 crossing over can lead to more adaptive re- 

 combinants, such inversion heterozygotes are 

 at a disadvantage compared to noninversion 

 or inversion homozygotes because of the ab- 

 sence of recombination among genes within 

 the inverted region. Nevertheless, very 

 small inversions do survive in many species. 



Consider the meiotic process in heterozy- 

 gotes for larger paracentric inversions. In 

 this case (Figure 12-3A), synapsis between 

 homologs occurs for all regions except those 

 adjacent to the points of breakage. This 

 synapsis requires one homolog twisting in 

 the inverted region while the other does not. 

 The figure happens to show the inverted and 

 not the noninverted chromosome twisting, 

 but the reverse is equally likely to occur. 

 If crossing over occurs anywhere outside the 

 inverted region, each of the four meiotic 

 products will be eucentric (having one cen- 

 tromere), as usual. If, however, a single 

 crossing over occurs anywhere within the 

 region inverted — as shown between C and 

 D — the two noncrossover strands of the 

 tetrad will be eucentric (one with and one 

 without the inversion), and the two cross- 

 overs will be aneucentric (having more than 

 one centromere or none). One of the aneu- 

 centrics will be acentric (duplicated for A 

 and deficient for G.HIJ); the other will be 

 dicentric (deficient and duplicated for these 

 respective regions). If the inversion is only 

 moderately long, only one crossing over can 

 occur within it; if sufficiently long, double 

 crossing over is possible. When such double 

 crossing over is of the two-strand type, both 

 crossover strands are eucentric. 



In animals, gametes function regardless 

 of the ploidy of the meiotic products they 



