382 Action of the Genetic Material 



and penetrance are positively correlated, which could also be stated 

 by saying that the expressivity is more stabilized with higher pene- 

 trance. It seems that this feature of penetrance is not based upon the 

 genie action assumed to hold for all the other quantitative effects. 

 This can best be seen by a comparison with similar facts involving 

 dominance. A good example is the scalloping eflFect upon Drosophila 

 wings produced by different mutant loci. In some cases (e.g., cut, 

 Xasta) tlie effect is perfectly symmetrical. In others, like the vestigial 

 alleles, it is more or less asymmetrical, though never as extreme as in 

 the homoeotic mutants which we studied, where frequently one side 

 is completely normal. A good case can be made out for the dominant 

 Beaded, where the effect is always asymmetrical, but I found a 

 modifier (or group of modifiers) which makes the efi^ect perfectly 

 symmetrical (not published). This shows that the asymmetry is not 

 one of the features of action of the mutant locus but of some inde- 

 pendently determined condition of the reacting system. Mather 

 (1953a) speaks of chance disturbances of development. In view of the 

 kind of correlations between penetrance and expressivity (negative 

 except at the extreme end) and between penetrance and symmetry 

 (positive), in our case not chance events but definite characteristics 

 of determination must be the cause of the asymmetries. The same can 

 be demonstrated also for incompletely penetrant loci like podoptera, 

 in which there are lines with varying degrees of asymmetry without 

 any visible rule; but there are also lines in which, for example, a 

 majority of individuals show the effect, even the most extreme one, 

 on one side only (see pod K for details). This indicates that inde- 

 pendent genetic conditions are involved, which may have to do with 

 the early distribution of cleavage nuclei in the peculiar development 

 of insects, with its migration of the cleavage nuclei within a common 

 cytoplasm. Thus, asymmetric effects must be appraised in regard to 

 their meaning for genetic action in each individual case. (For more 

 details, see our podoptera and tetraltera papers.) 



We see that the facts of penetrance give us the same information 

 on genie action as all the other data reported in this section, but they 

 emphasize more the threshold concept and the possibility of fluctu- 

 ation of the products of reaction near the threshold. They show us, in 

 addition, the interplay with other independently determined develop- 

 mental processes, especially those concerned with time and progress 

 of final determination of embryonic parts. 



