235 



FIGURE 27-4. The variability of nurnial corn is being pointed out by Jamls h. Crow 

 {Photographed in 1959 by Tlie Calvin Company.) 



that natural populations contain certain 

 paracentric inversions. When laboratory 

 populations are initiated, containing some 

 individuals with the normal chromosome ar- 

 rangement and others with a particular one 

 of these inversions, in some cases, the popu- 

 lation comes to contain only normal chromo- 

 somes after a number of generations has 

 passed. In such cases, the inversion chromo- 

 some behaves like a detrimental gene which 

 provides no advantage when heterozygous, 

 and is ehminated from the gene pool. When 

 other particular inversions are tested this 

 way, however, an equilibrium is reached in 

 which the normal and inverted chromosomes 

 are both retained in the gene pool. In these 

 cases, the inversion heterozygote is adaptively 

 superior to either homozygote, showing 

 heterosis, just as does the gene for sickling 

 in malarial countries. It is not easy to de- 



cide the genetic basis for heterosis in such 

 cases, however, since hybrid vigor could be 

 due to the genes gained or lost at the time the 

 inversion was initially produced, or it could 

 be due to position effect, or to individual 

 genes or groups of genes contained within 

 the inverted region. (It should be recalled 

 that individuals with paracentric inversions 

 are not at a reproductive disadvantage in 

 Drosophila. Should a heterotic system exist 

 or develop in paracentric inversion heterozy- 

 gotes, based upon the action of several genes 

 contained within the inverted region, this 

 heterotic arrangement would tend to remain 

 intact in the heterozygote, because of the 

 failure of single crossovers within the in- 

 verted region to enter the haploid egg 

 nucleus.) 



Finally, it would be appropriate to de- 

 scribe briefly how hybrid vigor has been 



