CHAPTER XXII 



INHERITANCE OF SIZE AND OTHER QUANTITATIVE 

 CHARACTERS. THE HYPOTHESIS OF MULTIPLE FACTORS 



Having observed how wide-spread unit-character variations 

 are and what an important part they play in the formation of 

 varieties of domesticated animals and cultivated plants, it is 

 natural to inquire whether any other sort of heritable varia- 

 tions occur, whether in the last analysis all inheritance is 

 Mendelian inheritance. This view is held by many students 

 of genetics at the present time. The cases of doubtful inter- 

 pretation relate chiefly to variations in size or shape of the 

 organism or of its parts, cases in which the characters under 

 observation vary continuously. 



That size may be affected by ordinary Mendelian factors 

 has never been questioned. One of the seven unit-character 

 variations studied by Mendel himself was found in the cross 

 between tall and short varieties of peas. Tall was found to be 

 dominant and the alternative conditions, tall and short, were 

 observed to segregate in true-breeding types in F2. In man 

 brachydactylism was early demonstrated to be a dominant 

 unit-character, by Farabee confirmed by Drinkwater. In 

 this peculiar condition, the skeleton is shortened throughout, 

 and in particular the fingers are reduced from the usual 

 three-jointed to the short, two- jointed condition. An analo- 

 gous variation in Drosophila known as ''dachs" is inherited 

 in the "second chromosome" group of genes. 



But the ordinary size differences between races of men, 

 breeds of animals, or varieties of plants, are not inherited in 

 this simple way, with dominance of one type, followed by 

 complete segregation from an alternative type. As a rule 

 intermediates or blends are produced in Fi (see Fig. 130). In 

 F2 the commonest type is still the intermediate as in Fi, but 

 variability is considerably increased, which may be regarded 



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