PART II. GROWTH AND MORPHOLOGICAL CHARACTERS. 



12. INTRODUCTORY DISCUSSION. 



The success of Mendel's experiments, which led to the discovery of 

 his "law of dominance and segregation," was due in a great measure to 

 the fact that his materials and methods were well chosen. The char- 

 acters dealt with were simple and well defined. Previous workers had 

 tried to follow too many characters at one time, or characters with 

 much fluctuation. The early work of those who first sought to cor- 

 roborate Mendel's experiments dealt with relatively simple characters. 

 The scope of work on inheritance broadened out in due time, and more 

 complex cases were studied, solved, and interpreted in accordance with 

 the theory of alternative inheritance. From time to time complete 

 lists of the various Mendelizing characters have been published, showing 

 the wide range of applicability of Mendel's law. Numerous experi- 

 ments indicate that the factors for a pair of allelomorphic characters 

 segregate from each other in gametogenesis and recombine in fertili- 

 zation according to the laws of probability. This hypothesis is accepted 

 on the evidence of the behavior of visible characters in crosses, for 

 segregation of a dominant factor from its recessive mate would give 

 in certain crosses a distribution of somatic characters in classes accord- 

 ing to the formula (S-f-l)" (where n equals the number of allelo- 

 morphic pairs). Actual results agree with this theoretical interpreta- 

 tion. Many characters, however, do not lend themselves to such a 

 simple solution. The inheritance of many size-characters is a matter 

 of much contention. Some maintain that a cross between two indi- 

 viduals differing in size or in a particular size-character may result in 

 a real blend. Others assert that the inheritance of size-characters is 

 essentially Mendelian, or is susceptible of such an interpretation. It 

 can, by no means, be considered that the question of size-inheritance 

 is settled. 



In the case of Mendel's peas, tallness and dwarfness were found to 

 be an allelomorphic pair. Other similar cases in plants are well known. 

 The abnormal shortness of bones and general stature in cases of brachy- 

 dactylism (Farabee 1905, Drinkwater 1908) is inherited alternatively. 

 In order to study size inheritance advantageously, it is quite necessary 

 to have two parent races which breed true to their particular size- 

 character. The absolute difference between the parental characters 

 should be large enough to admit of no confusion. The range of vari- 

 ability for each character should be such that they do not overlap. 

 Environmental influences should not obliterate the difference between 

 the races. The coefficient of variability for each parent race should be 

 small. However, animals which are adapted to genetic experimenta- 

 tion have met in very few cases these essential conditions. Therefore 



47 



