176 I The Process of Evolution 



ous that, with meiotic drive, an allele might increase in a population 

 even if it had a deleterious effect. In such a case, selection presum- 

 ably would result in the accumulation of factors that reduce the 

 effect of the gene responsible for meiotic drive. It is possible that 

 meiotic drive might result in the spread of a beneficial gene. The 

 extent to which this has occurred in nature is unknown. 



CHANGES IN CHROMOSOME STRUCTURE 



The usual cytological mechanisms of haploid and diploid organisms 

 already have been described. The student should review the ma- 

 terial on chromosome behavior in Chap. 3 before reading the fol- 

 lowing discussion of derivative genetic systems, those in which 

 cytological changes operate to increase or ( more often ) to decrease 

 what usually is thought of as the standard amount of recombination. 



Inversions 



Chromosome inversions are structural changes that can lead to 

 supergene formation. Here a section of the chromosome is in re- 

 verse sequence as compared with a standard. Upon synapsis in 

 meiosis, one of the pairing chromosomes must become twisted, and 

 a characteristic loop is formed in an organism heterozygous for an 

 inversion. As has been seen, crossing-over and recombination are 

 profoundly affected. Single crossing-over within the loop of a para- 

 centric inversion (one that does not include the centromere) leads 

 to the formation of a dicentric chromatid and a chromosome seg- 

 ment without a centromere. At anaphase I, these appear as a bridge 

 connecting the daughter nuclei and a fragment which does not move 

 to the poles (Fig. 3.2). Depending upon the number of the cross- 

 overs and the chromatids involved, all or a portion of the gametes 

 are unbalanced genically and therefore nonviable. Thus the genes 

 within the inversion remain together as a supergene that is the 

 length of the inversion. The gametes containing recombinations of 

 the genetic code within this supergene may not survive, or the 

 resultant zygote may be inviable. One way or the other, recombi- 

 nants are not produced. 



In a long inversion, more than one crossover may occur; the results 

 depend upon whether the same or different chromatids are involved 

 in the second as were concerned in the first. If the same chromatids 

 are affected, the second crossover will compensate for the first and 

 there will be no detectable cytological effect. If a new pair cross 

 over, a double bridge and two fragments are found at anaphase I. 



