INVESTIGATIONS SINCE MENDEL 



551 



due to the absence of the factor for tallness. Since the presence 

 and absence hypothesis explains more cases than the dominant 

 and recessive hypothesis, it has been generally accepted. 



Again Mendel worked chiefly with qualitative characters, which 

 have been found to behave differently from most quantitative 

 characters, such as size and weight. For example, in crossing large 

 and small varieties of Corn, the individuals of the first hybrid gen- 



FIG. 480. Inheritance of length of ears in Corn. The ears Pi are ears 

 of the parent plants (Tom Thumb Pop Corn at the left and Purple Flint Corn 

 at the right) chosen to represent the average length of ears of parents. Notice 

 that the ear of the F\ generation is intermediate in length between the paren- 

 tal ears, while in the F 2 generation, as shown by the ears at the left and right 

 of the F i ear, the length of ears range from that of Tom Thumb Pop to that 

 of Purple Flint. After East. 



eration are intermediate in size between the parents, the size of 

 neither parent dominating, and in the second hybrid generation 

 the individuals are of various sizes, ranging from that of the 

 smaller to that of the larger parent (Figs. 479, 480 and 481). 

 At first such cases were considered striking exceptions to Men- 

 del's law. However, a more careful study has led to the view 

 that quantitative characters do mendelize but commonly depend 

 upon so many independent factors, each of which is responsible 



