CHAPTER V 

 NEO-MENDELISM 



Thus far we have been considering Mendel's law in 

 its simple form and have enlarged but little upon 

 MENDEL'S original statement. The value of the law is 

 apparent. Upon its republication in 1900 it was taken 

 up by biologists and numerous breeders set to work to 

 test it. As a consequence data for and against it began 

 to accumulate. As might be expected, there was much 

 apparent evidence against the law, but as geneticists 

 developed a better conception of the mechanism the 

 contradictory evidence was explained away. Almost 

 every type of inheritance has now been explained 

 according to Mendel's law. Some of the explanations 

 are very complicated and cannot be included in this 

 presentation. A few of the more important cases, how- 

 ever, will be presented. 



I. PRESENCE AND ABSENCE HYPOTHESIS 



This may be regarded as a new method of Mendelian 

 thought. It was first suggested by CORRENS (3), but 

 later was worked out in detail by other geneticists, 

 especially HURST, BATESON, SHULL, and EAST. It is 

 merely a modification of the mechanism involved. For 

 example, in the case of a hybrid obtained by crossing tall 

 and dwarf parents the result had been explained as due 

 to the fact that one chromosome bears a determiner for 

 tallness and the other one of the pair carries the deter- 



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