GENIC INTERACTION 



7 77 



threshold would be rare. Because one 4- toed had been obtained in Fj (32 x D), a 

 cross which gave results very close to those 2 x D in F 2 and the backcross to D, the 

 mean of (2 x D) was put only 2.5a below the lower threshold. The means of the pure 

 strains 2 and 13 were located on the hypothesis that F x in each case was exactly half 

 way between the parental strains on the physiologic scale. 



Fig. 28. Factors determining development of the small toe. 



3-toe' 

 2 13 35 



poor I good 

 4-toe 1 4-toe 



(2*D) 2 03xD) 2 (35xD) 2 



-9 - 



Theoretical distribution of factors determining development of the small toe on an 

 underlying physiologic scale in relation to thresholds for any small toe and for perfect 

 development, in four inbred strains, in F x and F 2 of crosses of one (D) with the other three. 



The relations of variability in F 2 (2 x D) to the interval between the means of 

 these two most extreme strains permit a minimum estimate of the number of gene 

 differences, assuming equal effects and no dominance. The number is approximately 

 four. The hypothesis of one major factor, responsible for half the difference and 

 practically all of the variance plus a great many minor factors, fits equally well. So 



