155 



G H I J 



A. PARACENTRIC INVERSION 



A B C D 



I J 



B. PERICENTRIC INVERSION 



gametes is suppressed in the paracentric in- 

 version heterozygote female either because 

 of nonsynapsis, or, in cases of synapsis, be- 

 cause of the exckision from the egg of cross- 

 overs within the inverted region. 



What is the effect of a chiasma within 

 the inverted region in a heterozygote for a 

 larger pericentric inversion? As seen in Fig- 

 ure 19-3B, a single chiasma, such as between 

 F and G, produces four eucentric strands: 

 two are noncrossovers (one with, and one 

 without, the inversion, these being, again, 

 those strands of the tetrad not included in the 



FIGURE 19-3. A single chiasma in an inversion 

 heterozygote. {See text for explanation.) 



Figure), one has a duplication (for ABCD) 

 and a deficiency (for IJ), the other has a 

 deficiency and a duplication of these same 

 regions, respectively. These two aneuploid 

 strands will enter the gametes of males, if 

 crossing over occurs in the male. They will also 

 be present in the gametes of females that have 

 crossing over, even in the case of Drosophila, 

 since all meiotic products are eucentric and 

 there is, therefore, no shunting of euploid 

 strands into the egg nucleus. So, the aneu- 

 ploidy resulting from crossing over within a 

 pericentric inversion always puts the hetero- 



