.Murgan, Are the Germ-Cells of Mendelian Hybrids "Pure"'? 293 



For example; if an extracted recessive W(G) is crossed with 

 B, the offspring will be WGB, i. e., gray. The outcome must 

 be accounted for by the presence of the gray in W(G). On 

 the other hand if an extracted dominant G(W) is cros- 

 sed with B, the offspring is gray because this color dominates 

 both black and white. Similarly if the extracted dominant, B(W), 

 is crossed with G the offspring is gray; or if G(B) is crossed with 

 white still the gray dominates. In all combinatioris, therefore, with 

 the extracted dominant it is impossible to bring out the latent 

 color in the first generation for the obvious reason that it is domi- 

 nated by the dominant; but in "the second generation, that results 

 from inbreeding, it should be possible to demonstrate the presence 

 of the latent color of the grandfather who was an extracted 

 dominant. For example: the extracted dominant G(W) crossed 

 with B, as above, gives the gray mouse GWB, whose germ-cells 

 would be G(WB), W(GB), B(GW) which will produce gray, black, and 

 white mice, in the following proportion 1 G, 2 GW, 2 GB, 2 BW, 

 1 B, 1 W. Since the first three terms of this series are gray 

 mice, and the next two black mice, and the last only white, there 

 are 5 grays, three blacks, and one white. The white must owe 

 its origin to the color of the extracted dominant. The number 

 of white mice expected in this combination is quite small and a 

 large number of offspring of this generation might be required to 

 demonstrate its occurence. By making another combination, how- 

 ever, it is possible to increase the expectation. If, for instance, 

 we start with an extracted dominant G(B) and cross it with a 

 white mouse, we get a mouse GWB, whose germ-cells will be 

 G(BW), B(GW), W(GB). These will give the sarae kinds of young 

 as for the case just given; and in the same proportion, but since 

 the black color is here derived from the latent color of the extac- 

 ted dominant there will be three mice of this color to five grays 

 and one white. 



In the literature I know of one case that seems to show 

 something of this kind. Darbishire 1 ) found that when ex- 

 tracted dominants (pink eyed mice with spotted coats) were 

 crossed with white mice, some of the offspring were white, 

 which is not the expectation if the germ-cells of the extracted 

 dominant are pure; for, they should give the same result as when 

 individuals of the pure strahl are crossed with albinos. But these 

 do not give any white! It should also be noted that the ex- 

 tracted dominants, when inbred, gave one albino to thirty-two spotted 

 individuals showing that while the extracted dominants approached 

 the condition of breeding true, they did not do so invariable. 



1) Biometrica. 3. Jan. 1904. 



