362 Action of the Genetic Material 



do we intend to criticize the entire concept, though the unlimited 

 accumulation of impossible numbers of loci is certainly a strain upon 

 the imagination when the concept is applied to all quantitative and 

 selectable characters of an organism. Among those who made experi- 

 ments similar to those of Mather, but who reached very different 

 conclusions, Robertson and Reeve (1952), and Reeve and Robertson 

 (1953) may be mentioned. They assume that pleiotropy and genie 

 interaction are decisive factors for variation in quantitative characters. 

 Thus selection effects may be due to influences upon the genetic 

 background, which shifts those genie effects. In this case a few major 

 and some minor genes would suffice for explaining the selection re- 

 sults. Apart from the problem of selection, the experiments of these 

 authors lead to the suggestion that the entire range of variability of 

 the character in the inbreeding and selection experiments may be 

 explained by only three or four pairs of genes with a major effect. I 

 have reached a similar conclusion, as did S. Wright (I9'34b, 1935) 

 in his experiments on selection of Polydactyly; and, in some instances, 

 I could isolate individual loci the action of which could be measured 

 ( see Goldschmidt, 1953a ) . Thus there will be cases in which quantita- 

 tive inheritance may act on a simple (but not necessarily primary) 

 dosage principle. (Further discussion of quantitative inheritance is 

 not needed for the problems of this chapter, since the major aspects 

 of the phenomenon are involved in the problems of selection and not 

 of genie action.) 



Griineberg ( 1952a ) has recently discussed the problem of quanti- 

 tative inheritance from the point of view of physiological genetics, 

 and has reached conclusions which we might have mentioned in con- 

 nection with penetrance and threshold effects. He worked with a mu- 

 tant in mice in which the third molars are absent. The abnormals ap- 

 pear in certain litters in bunches, which indicates an environmental 

 influence upon penetrance. In the same group, the number of molars 

 missing per individual is increased; that is, higher penetrance pro- 

 duces higher expressivity. Though the anomaly is dominant, it hardly 

 segregates in outcrosses. The explanation of this is derived from the 

 fact that in the abnormal lines the existing third molars are smaller 

 and more variable than in normal lines; this feature shows the same 

 characteristics in regard to expressivity and bunching as the absence 

 of the tooth does. The character, reduced molars, has multiple factor 

 inheritance, and complete absence is only the end of a series of 

 genetic variation. Thus the "discontinuous" variant, absence, is a part 

 of "continuous variation." A threshold condition separates the quasi- 



