Structural Changes in Chromosomes 



169 



A. PARACENTRIC INVERSION 



iA 



.A«B"C — D 







B. PERICENTRIC INVERSION 



figure 12-3. A single crossing over in an 

 inversion heterozygote. {See text for explana- 

 tion.) 



contain. If a gamete contains an aneucentric 

 produced by crossing over in a paracentric 

 inversion heterozygote, this chromosome will 

 usually have a dominant lethal effect after 

 fertilization; that is, individuals heterozygous 

 for moderate to large paracentric inversions 

 are at reproductive disadvantage, which often 

 leads to the elimination of the inversion from 

 the population soon after it arises by muta- 

 tion. This disadvantage is partly avoided in 

 those species having no crossing over in one 

 sex. For example, in the Drosophila male 

 each homolog, inverted or not, is a noncross- 



over and has the same chance of being in- 

 cluded in the gamete. A special factor oper- 

 ates during meiosis in some species in which 

 the female undergoes crossing over, occur- 

 ring only if the two meiotic divisions occur 

 in tandem, as they do in female Drosophila. 

 In the Drosophila oocyte heterozygous for 

 a paracentric inversion, a single crossing 

 over within the inverted region produces the 

 usual dicentric at anaphase I. But this di- 

 centric serves to hold the dyads at metaphase 

 II, so that the two eucentric monads proceed 

 to the two outermost of the four poles. 

 Therefore, at the end of telophase II, the cen- 

 tric meiotic products are arranged in a row: 

 eucentric; part of dicentric; remainder of di- 

 centric; eucentric. It is one of the two end 

 eucentric-containing nuclei which becomes 

 the egg nucleus, the others degenerate. In 

 this way the dicentric strand is prevented 

 from entering the nucleus that becomes 

 gametic; the gamete, therefore, receives one 

 of the two eucentric, noncrossover strands. 

 That is why in Drosophila, paracentric in- 

 versions of any size rarely cause aneuploid 

 gametes in either sex and can become estab- 

 lished in nature. 



What products result from a crossing over 

 within the inverted region in a heterozygote 

 for a larger pericentric inversion? As seen 

 in Figure 12-3B, a single crossing over, such 

 as between F and G, produces four eucentric 

 strands: two noncrossovers (one with and 

 one without the inversion) ; one with a dupli- 

 cation (for ABCD) and a deficiency (for 

 IJ) ; the last with a deficiency and a duplica- 

 tion of the respective regions. All strands 

 enter the gametes of males if crossing over 

 occurs in the male. Each strand also has 

 an equal chance of being present in the 

 gametes of females capable of crossing over. 

 This is true even in Drosophila where shunt- 

 ing of euploid strands into the egg nucleus 

 does not occur because all the meiotic prod- 

 ucts are eucentric. Consequently, aneuploidy 

 which results from crossing over within a 



