MORE RECENT INVESTIGATIONS OF MENDELISM 551 



mals. By working according to the Mendelian conception, 

 many desirable varieties of the cereals, much more desirable 

 ornamental plants, and various kinds of better fruits have been 

 developed. 



Of course in the extended investigations in Genetics since 

 1900, many situations have arisen that Mendel did not meet. 

 Cases have arisen in which the Mendelian behavior can be 

 explained better by assuming that pairs of contrasting characters 

 are due to the presence and absence of certain factors and not to 

 dominant and recessive factors. According to the latter hypothe- 

 sis, the tallness of the tall variety of Peas is due to the presence 

 of a factor for tallness, while dwarfness in the dwarf variety is 



, FIG. 480. Height of plants in the F 2 generation of Tom Thumb Pop 

 Corn (a dwarf Corn) crossed with Missouri Dent (a large Corn). The plant 

 at the extreme right is similar in height to the dwarf parent, while the one at 

 the extreme left is similar in height to the Missouri Dent. After Emerson and 



East. 



due to the absence of the factor for tallness. The presence and 

 absence hypothesis explains some cases more satisfactorily than 

 the dominant and recessive hypothesis. 



Again Mendel worked chiefly with qualitative characters, which 

 have been found to behave differently from most qualitative 

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

 large and small varieties of Corn, the individuals of the first hybrid 

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



