CHAPTER IX 



MENDELIAN HEREDITY AND ITS MECHANISM 1 



I. The scientific era of the investigation of heredity 

 begins with Mendel's paper on plant hybridization 

 which was not appreciated by his contemporaries. 

 Mendel invented a method for the quantitative study 

 of heredity which consisted essentially in crossing two 

 forms of peas differing only in one well-defined heredi- 

 tary character; and in following statistically and 

 separately the results of this crossing and that of the 

 inbreeding of the second and third generations of 

 hybrids. This led him to the recognition of one 

 essential feature of heredity; namely, that while the 

 hybrids of the first generation are all alike, each hy- 

 brid produces two types of sex cells in equal numbers, 

 one for each of the pure breeds which has been used 

 for the crossing. This takes place not only when the 

 forms used for the crossing differ in regard to one 



1 For the literature on the subject the reader is referred to Morgan, 

 T. H., Sturtevant, A. H., Muller, H. J., and Bridges, C. B., The 

 Mechanism of Mendelian Heredity. New York, 1915. 



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