Genetic Systems II 1 177 



Various other configurations may occur, depending upon the number 

 and position of crossovers. Should an inversion be pericentric (i.e., 

 include the centromere), a supergene will be formed, but the inver- 

 sion is not detectable by a bridge and fragment as with paracentric 

 in\ersions. 



The evolutionary effect of inversions depends upon whether they 

 are relatively long or short. Short or moderately long inversions fre- 

 quently are found "floating" in populations of plants and animals 

 where they appear spontaneously. They reach high frequency when 

 they become associated by chance with a favorable combination of 

 genes. The genus Drosophila provides numerous examples of this 

 (see Chap. 7). It has been found that, concomitant with inversions 

 regularly associated with certain chromosomes, there may be marked 

 increase of crossover frequency in other chromosomes. Thus the 

 effect of the inversion must be to form supergenes rather than 

 simply to reduce the amount of recombination. For if selection were 

 operating to reduce recombination generally, the increase in cross- 

 over frequency in other chromosomes probably would not be ob- 

 served. 



When individuals from geographically isolated populations or 

 from different species are crossed in the laboratory, they are often 

 found to differ by very long inversions (perhaps of an entire arm). 

 Several such inversions prevent the formation of fertile offspring; 

 thus they may be thought of as isolating mechanisms. Their impor- 

 tance in nature is not as clear-cut as that of shorter inversions. It 

 seems clear that spontaneously occurring inversions might rather 

 rapidly differentiate karyotypes of populations that become sepa- 

 rated. When they occur with regularity, as in Drosophila and Sciaro, 

 they seem to be playing a special role. 



Reciprocal Translocations 



Reciprocal translocations likewise may function in setting up super- 

 genes in populations heterozygous for the change. If the inter- 

 changed segments are short, usually only two competent combina- 

 tions can be formed in a heterozygous organism. Should crossing- 

 over occur between the centromere and the point of interchange, 

 only two of the four chromatids associated at any one place may be 

 separated as competent combinations (see Chap. 3). With more than 

 one interchange between the same two chromosome pairs, rings of 

 more than four chromosomes are formed at meiosis. In some organ- 

 isms, such as the plant Rhoeo and several species of Oenothera (eve- 

 ning primrose), all the chromosomes exchange segments and all are 



