60 A MANUAL OF MENDELISM 



factors operating and that there should be four groups 

 in the hybrids' progeny, as follows : 



Instead, however, the hybrids' progeny consist of only 

 three kinds in a proportion which is entirely new, namely, 

 adding both the above results together, 



with the further deviation from the usual Mendelian result 

 that the largest group contains no true-breeding individual 

 while every individual in the other two groups breeds 

 true. To meet these phenomena, a new hypothesis or, 

 at any rate, a modification of the ordinary Mendelian 

 hypothesis is necessary. It will be remembered that, 

 ordiaarily, when the parents differ in one pair of characters, 

 say X and x, the hybrids' progeny consist of two groups, 

 the larger of which can be divided into hybrids and true- 

 breeding individuals, while the smaller contains true- 

 breeding individuals only, thus : 



Hybrids Breeding Breeding 

 true true 



