290 Morgan, Are thc Germ-Cells of Mendelian Hybrids "Pure"? 



dominating characters of the other. For instance, an albino niouse 

 crossed with a gray gives gray young. These inbred give some 

 grays and some wliites. The latter (the extracted recessives) eon- 

 tain gray "blood", which can be demonstrated by crossing them 

 with black mice, when gray offspring will be produced. Now, on 

 the generali} 7 accepted view, the extracted recessive, the white 

 in this case, has been formed by the union of a pure, white- 

 bearing Spermatozoon with a pure, white-bearing egg, but this 

 Interpretation obviously fails to account for the appearance of the 

 gray color when the extracted white mouse is crossed with a black. 

 As a matter of fact it has been recognized in recent years by 

 several investigators that the extracted recessives do carry the 

 characters of both grandparents, although the dominant character 

 has been supressed in such a way that it usually does not appear 

 as long as the extracted recessives are inbred, but reappears when 

 the extracted recessives are crossed with mice of other colors. 

 My contention is, that, if this is the case, the germ-cells of the 

 hybrid are not pure, and that if the extracted recessives are im- 

 pure, the extracted dominante by parity of reasoning are also im- 

 pure in the same sense. I have suggested how we can account 

 for the Mendelian proportion by another assumption, namely, by 

 that of alternating dominance and latency of the coutrasted cha- 

 racters in the germ-cells of the hybrid. 



My object in writing this second paper is, first, to clear up 

 a point in regard to the extracted dominant which is insufficiently 

 developed in my former paper, although fully implied in the for- 

 mulae there given, and, in the second place, to point out certain 

 possibilities for further work, that are not apparent, on the cur- 

 rent Mendelian assumptions. Lastly the condition of Cuenot's yellow 

 mice must be given further consideration. 



Let me recall the main point in my former paper in regard 

 to the two kinds of germ-cells of the hybrid. If wo obtain a 

 hybrid by crossing a white, W, and a gray, G, mouse, all of its 

 cells will eontain both the white and the gray characters; the 

 latter dominating the former. Whether these characters are re- 

 presented in the chromosomes of the cells, or in the protoplasm 

 is immaterial for the present point of view. The germ-cells of 

 the hybrid will eontain, at first, these two sets of contrasted cha- 

 racters, but at some time in their history, one of the two con- 

 trasted characters becomes dominant, the other becoms latent. 

 These germ-cells may be represented by the formulae G (W) and 

 (G) W, indicating that in the one case the white, (W), and in the 

 other the gray, (G), character has become latent. If some 

 mechanism exists in the egg by which equal proportions' of these 

 two kinds of germ-cells are formed, the Mendelian proportion will 



