380 Action of the Genetic Material 



the present discussion ) . In other lines with a much higher penetrance 

 it is possible to isolate the individual multiple factors within their 

 chromosomes and to state that each one alone produces a certain share 

 of the penetrance, which is different for the different loci. The com- 

 bined effect of these was found to be sometimes additive, sometimes 

 multiplicative. In special cases it was possible to accumulate a suflB- 

 ciently large number of such loci (or sufficiently potent ones) to 

 accomplish a 100 per cent penetrance. When this was done by 

 selection, it was accomplished by finding a few very potent loci 

 rather than by accumulation of many loci of low potency, which 

 never led very far. In principle, then, penetrance may be treated here 

 like any phenotypic effect which is quantitatively increased by an 

 accumulation of mutant loci of individually insufficient quantitative 

 action (i.e., like any effect of multiple factors), and therefore the dis- 

 cussion of these in regard to genie action (see III 5 C a dd) applies 

 also to penetrance. The only difference is that in multiple growth or 

 color factors the explanation is that different quantities of an end 

 product are the result of the genie actions, that is, more growth sub- 

 stance or pigment; while in the present case there must be an ad- 

 ditional feature, namely, a threshold level for normality. The analysis 

 of the details of the effect shows that this threshold condition is set 

 by the time in development at which genie action comes into play, as 

 will be discussed below. 



2. Penetrance is affected by genetic modifiers, which themselves 

 are not in control of the same phenotype (as opposed to multiple 

 factors). As in all cases of modifier action, either the modifiers have 

 no other known functions, and may be demonstrated solely by the 

 positive effect of selection experiments; or they may be loci of known 

 visible action, frequently dominants, which in this case act as specific 

 modifiers. This again shows penetrance to be a consequence of the 

 same general type of genie action as that which accounts for the 

 explanation of all modifier action. Many examples are found in 

 the papers on podoptera. 



3. Penetrance is more or less influenced by temperature condi- 

 tions, which may shift penetrance more or less in individual cases. 

 This applies to penetrance of single mutants, for example, in Timo- 

 feeff's work, and also to the multiple factor penetrance in podoptera. 

 The details are not different from the temperature action, say, upon 

 the number of Bar facets or the ci effect or any other mutant action 

 which has a temperature dependence, which again means that it must 

 be based on the same dynamic, kinetic system of genie action. 



