I HO PHYSIOLOGIC GENETICS 



just as multiplicative constancy can be converted into additive constancy by the use of 

 logarithms. Such transformations are often useful but usually break down more or 

 less in more complicated systems. 



No effect in some combinations. — The frequent reduction of the F 2 ratio 9:3:3:1 to 

 9 : 3 : 4 or to 12:3:1 indicates the frequency of specific modifiers of the effect of a domi- 

 nant or of a recessive gene, respectively. Thus in guinea pigs carrying EBF, CP is 

 black, Cpp is pale sepia, but either c a c a P or c a c a pp is white. In those carrying EBC, 

 either PF or Pffis black, ppF is pale sepia, and ppff is pale brownish cream. The effect 

 of M, m in rough (R) and smooth (rr) guinea pigs illustrates the case of a specific 

 modifier of a dominant that itself lacks dominance (Rmm, full rough; RMm, inter- 

 mediate rough; RMM, rough only on the toes; but rrMM, rrMm, and rrMM all 

 equally smooth). 



There may be mutual instead of one-sided dependence as in the cases of comple- 

 mentary dominants (F 2 ratio 9:7), complementary recessives (F 2 ratio 1 5 : 1 ) , or comple- 

 mentary dominant and recessive (F 2 ratio 13:3). In guinea pigs, CP gives dark eyes, 

 but Cpp, c a c a P, and c a c a pp are all pink-eyed (although Cpp can often be distinguished by 

 traces of color). With sisidmdm, guinea pigs are anemic, spermatogenesis is absent in 

 the diminutive testes of males, and fecundity is low in females, whereas with either Si 

 or Dm or both, these are normal. There are, however, cumulative effects of Si, si, 

 Dm, dm on intensity of color. 



All such two-factor cases can be treated as multiplicative by assigning zero to the 

 effect of one member of one or both pairs of alleles and choice of a suitable scale, but 

 this sort of constancy of effect cannot as a rule be extended to all combinations. 



Threshold and ceiling effects of a more complex sort may also be included in this 

 category. All of the colors of the guinea pig are subject to threshold effects. A ceiling 

 effect is most conspicuous in the case of the dark-eyed browns, especially when adult. 

 The irregular penetrance that is characteristic of most mutational effects on morphologic 

 characters implies a threshold. The occurrence and variations in degree of develop- 

 ment of the small toe give an example of a case in which there is homeostatic control 

 of development both below a threshold and above a ceiling. 



Opposite directions of effect in different combinations. — In guinea pigs with CPff, those 

 with E (black) have much more pigment than those with ee (dilute yellow) whereas with 

 Cppff, those with E (pale brownish cream) have much less pigment than those with ee 

 (same dilute yellow as in the preceding case). The intensifying effects of C, P, and F 

 in brown (Ebb) up to a certain point and their dilution effect beyond this point is 

 another example. In this case, Pp always shows overdominance, a phenomenon in 

 which the alleles necessarily reverse their order of effect in different compounds. 

 The increase in intensity after birth of c d sepias and yellows, in contrast with the decrease 

 in intensity after birth of c k sepias and yellows, illustrates reversal of effect of a non- 

 genetic condition in different genotypes. In this case c k sepia is darker than c d sepia at 

 both ages, and c k and c d yellows hardly differ at birth. 



In the case of comparison of intense browns (EbbCPp) and sepias of genotype 



