Genie Control of Development 381 



4. TimofeefiF-Ressovsky introduced simultaneously with pene- 

 trance the term "expressivity," meaning the degree of phenotypic 

 expression of a trait. It is possible that a mutant always (i.e., in all 

 individuals) produces the maximum possible effect; for example, a 

 white eye is always a white eye. Other mutants have a more variable 

 effect, so far as there is a considerable fluctuation in the degree of 

 the effect under identical conditions (i.e., in expressivity). In addition, 

 expressivity is frequently affected by modifiers or by temperature 

 action, sometimes within small limits, sometimes within the entire 

 range from normal to the extremest possible effect. It seems that 

 mutants with incomplete penetrance have a special tendency to 

 variable expressivity. Otherwise great irregularities are observed. We 

 described (Goldschmidt et al., 1951; Goldschmidt, 1953a) cases in 

 which no correlation between penetrance and expressivity was found, 

 and others in which there was no correlation up to high penetrance, 

 but complete correlation with the highest grades of penetrance. Bezem 

 and Sobels ( 1953 ) found complete correlation for a different, rather 

 extreme mutant of Drosophila, Asymmetric. Thus it may be assumed 

 that the type of genie action which accounts for penetrance also may 

 control expressivity, directly or indirectly, except for other interfering 

 reactions. Penetrance, like dosage, dominance, and potency, is probably 

 a rather direct result of the genie effect upon the kinetics of the chain 

 of reactions, together with specific threshold conditions for the action 

 of the reaction products; but expressivity must involve some additional 

 developmental features interacting with the reactions responsible for 

 different penetrance. Such a feature could be the time of competence 

 or labile determination with slowly progressing irreversibility (see III 

 5 D e). The detailed facts of the entire effect of such genie action 

 (i.e., a definite penetrance plus a certain expressivity) would thus 

 suggest a general model of genie action of the types used in figures 

 18 and 19, with the addition of another variable, the progress in time 

 of irreversible determination of the anlage. 



5. Incomplete penetrance is frequently combined with more or 

 less asymmetry of the effect, though the same may be found in certain 

 dominance effects also. Astauroff (1929) has shown that this asym- 

 metry is based upon independent development of the two halves of 

 the body (in Drosophila). This means that though right and left are 

 affected independently in regard to the expressivity of the character, 

 thus producing more or less, even extreme, asymmetry, neither side is 

 preferred (or has a bias, as Mather, 1953fl, expresses it in a statistical 

 study). Our own work bears this out, but we found that symmetry 



