MENDEL'S LAW OF DICHOTOMY IN HYBRIDS. 



C. B. DAVENPORT. 



IN the study of hybrids we must, as De Vries (I900b) truly 

 says, no longer pay primary attention to the degree of differ- 

 ence between the forms united - - to whether they are species, 

 subspecies, or varieties-- but to the behavior of the peculiar 

 characters by which the crossed individuals and their ances- 

 tors are distinguishable. For each of these somatic characters 

 corresponds to some peculiarity of the germplasm. The be- 

 havior of the differing characters when united in the hybrid is 

 diverse ; three categories have long been recognized (Galton, 

 1888, p. 12). These are: (i) blending heritage, illustrated by 

 skin color in man ; (2) alternative heritage, illustrated by 

 human eye color ; and (3) mixed heritage, illustrated by the 

 piebald condition of the progeny of mice of different colors. 

 The law of dichotomy in hybrids applies only to the second 

 class, --alternative heritage, -- although it has recently been 

 brought forward by De Vries (1900) as the almost universal 

 law of inheritance in hybrids. The law itself was first enun- 

 ciated very clearly and completely by Mendel (1865) and 

 deserves to bear his name. The law was, however, forgotten. 

 It has been rediscovered independently by De Vries and by 

 Correns (1900), both of whom are able to add new evidence of 

 its validity (for alternative heritage !). 



In his illustration of Mendel's law, De Vries first classifies 

 hybrids into monohybrids, dihybrids, and polyhybrids, accord- 

 ing as their parents differed in one character only, or in two 

 characters, or in many characters. The case of inheritance in 

 monohybrids is the simplest, and will be first considered. 

 Mendel's and De Vries's investigations have established the 

 following principles : 



i. Of the two antagonistic peculiarities the hybrid exhibits 

 only one ; and it exhibits it completely, so as not to be 



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