GREGORY: EFFICACY OF MUTATION BREEDING 465 



ships of lethals, sublethals, vitals, supervitals, etc., are only imper- 

 fectly known even in Drosophila (7, 37). 



Numbers of loci which are carrying deleterious alleles are 

 thought to be maintained in various degrees and frequencies in 

 some sensitive equilibrium by breeding system as well as by external 

 environment. The fact that such a genetically variable population 

 presents, in the wild at least, a uniformity of wild type has been 

 discussed by Lerner (28). The widely accepted explanation of the 

 coincident maintenance in the crossbred of genetic diversity and 

 phenotypic uniformity lies in the phenomenon of heterozygosis. 

 Thoday (52, 53) was able to show that heterozygosity was essential 

 for the preservation of bilateral symmetry in the fruit fly. 



The conclusion that mutation itself is in equilibrium with 

 requirements of the mating system derives from the simple genetic 

 situation that given a mutant A, advantageous in heterozygous 

 state, every other mutation which occurs affecting breeding sys- 

 tem will be selected in terms of A's conferred benefits. Furthermore, 

 that particular organization of the genome which permitted the 

 mutability of A will be favored in the sense that not only will those 

 forms possessing the A-mutating quality be favored in selection but 

 also those forms that possessed qualities controlling the A-mutating 

 frequency. The genome itself and its intrinsic characteristics are 

 conceived to evolve with breeding system to give the breeding 

 structure of the population. The cross-pollinator is thought to have 

 a genotype suited not only to meeting the contingencies of its 

 environment but to the controlling of the variability in its own 

 heterozygous organization. A measurable supply of new mutation 

 is steadily furnished the crossbred (47, 49). It is thought that the 

 rate of supply is determined by natural selection in keeping with 

 the demands of breeding system and environment. Such a dom- 

 inance-dependent organization would tend to neglect the evolu- 

 tion of high thresholds to expressivity for individual alleles. Any 

 radical change in breeding system, external environment, or muta- 

 tion rate would result in immediate shifts in the genetic composi- 

 tion of the population. 



The above considerations concern the efficacy of mutation 

 breeding with normally outbred organisms where the chief protec- 

 tion against deleterious mutation resides in heterozygosity. If selec- 



