78 Outline of Genetics 



The doctrine of cumulative factors was further devel- 

 oped by Emerson and East (5) in their work with corn. 

 They were able to explain some of the ratios obtained 

 by assuming three or four separately inherited cumula- 

 tive factors, just as Nilsson-Ehle had done. They 

 obtained other ratios, however, which required more 

 independent cumulative factors to explain. Some idea 

 of the extent of these investigations may be gained by 

 noting the list of plant characters whose inheritance they 

 explained on the basis of cumulative factors: number of 

 rows, length of ear, diameter of ear, weight of seed, 

 breadth of seed, height of plant, number of stalks per 

 plant, earliness of flowering. In all of these cases 

 breeding gave the same characteristic results. A cross 

 between extreme parents gave hybrid progeny inter- 

 mediate as to the character in question; and in the F2 

 generation the two extremes reappeared, along with all 

 gradations of intermediates. The relative frequencies 

 of these classes always resembled the normal probabili- 

 ties curve. 



Nilsson-Ehle had been able to put his F2 inter- 

 mediates into rather definite classes, corresponding to 

 the number of doses of the determiner each had received. 

 Emerson and East, however, could not do this with 

 such exactness. Their results showed all gradations, 

 but they could not distinguish any definite groups; 

 that is, gradation was continuous and complete. In 

 other words, they could not tell with certainty from out- 

 ward appearance just how many doses of cumulative 

 factors an individual contained. Their results, there- 

 fore, do not seem so clear and striking as those of Nils- 

 son-Ehle, but they are by no means vague and uncer- 



