DOMINANCE AND OVERDOMINANCE 297 



action known to be rare, but it is pointed out that if only a small proportion 

 of the loci are of this type, these may nevertheless be the major factor in the 

 population variance. 



The following interpretation is suggested: Inbreeding depression and re- 

 covery on crossing are mainly the result of loci at which the favorable allele is 

 dominant and the recessives are at low frequency. On the other hand the 

 variance of heterozygous populations and the differences between different 

 hybrids are due mainly to loci with intermediate gene frequencies. It appears 

 likely that such loci are due to selectively superior heterozygotes, but there 

 are several other possibilities. 



