CASTLE. MENDEL S LAW OP HEREDITY. 



539 



only. It bore only the recessive character. Of the yellow seeds, one 

 in three produced only yellow offspring, i. e. contained only the dominant 

 character • but two out of three proved to be hybrid, producing both 

 green and yellow seed, as did the hybrids of the preceding generation. 

 These are precisely the theoretical proportions, A -f 2 A(B) -|- B. 



The experiment has been repeated and confirmed by several different 

 observers. 



In mice, my friend and pupil, Mr. G. M. Allen, finds the second 

 hybrid generation, obtained by crossing gray with white mice, to con- 

 sist of gray mice and white mice approximately in the ratio, 3:1. 

 (See Table III.) The white are pure recessives, producing only white 

 offspring, when bred inter se. What portion of the grays are pure 

 dominants has not yet been determined, but we may confidently expect 

 that it will prove to be not far from 1 in 3. 



TABLE III. 



Heredity of Coat-color among Cross-bred Mice obtained by Mating 

 White Mice (W) with Gray Mice (G). 



A further test of the correctness of Mendel's hypothesis of the purity 

 of the germ-cells and of their production in equal numbers, is afforded 

 by back-crossing of a hybrid with one of the parental forms. For 

 example, take a case of simple dominance, as of cotyledon-color in peas 

 or coat-color in mice. We have here characters D (dominant) and R 

 (recessive). The first generation hybrids will all be D(R). Any one 

 of them back-crossed with the recessive parent will produce fifty percent 

 of pure recessive offspring and fifty per cent of hybrids. 



For the hybrid produces germ-cells .... D + R 

 The recessive parent produces germ-cells . . R + R 



The possible combinations are 2D(R) + 2R 



This case has been tested for peas and for mice and found to be 

 substantially as stated. 



