GENETIC STUDIES OF RABBITS AND RATS. 53 



skin which is yellow, and that a blastomere or blastomeres contain- 

 ing as normally both maternal and paternal members of this chromo- 

 some pair produced the gray part of the coat. 



But how about the germ-cells of this individual? Does the mixed 

 condition seen in the coat obtain also among the germ-cells? Very 

 likely it does. But if so, no tri-color progeny need be expected in 

 consequence. For those germ-cells which correspond in composition 

 with the gray parts of the coat would produce gametes like those of 

 any other Fi individual, which for the most part would transmit 

 either albinism or pink-eyed yellow, with an occasional gamete 

 transmitting gray and an occasional gamete transmitting both albin- 

 ism and pink-eyed yellow. But the germ-cells corresponding to 

 the yellow parts of the coat would transmit either pink-eyed yellow 

 or nothing (so far as the chromosome under discussion is concerned). 

 It is an open question whether this last type of gamete (lacking the 

 entire chromosome) would be viable, and if it were, its existence would 

 be difficult to detect, since it would behave Hke the double recessive 

 type of gamete, cp, in its effects on coat color. 



How, then, could a genuine genetically transmissible type of 

 mottled rat arise? Only, I suppose, by a change in the genetic 

 locus at P, p, so that it became, instead of P alone or p alone, a 

 mosaic Pp transmissible as a unit and capable of functioning as the 

 allelomorph either of P or of p. Such apparently is the case in the 

 guinea-pig, where Ibsen has shown the mottled condition to be the 

 allelomorph of both simple black and simple yellow. Such appar- 

 ently is also the condition in the mottled variety of rabbit called 

 "Japanese," which I have recently studied in a series of crosses, and 

 such would appear to be the case in maize with striped pericarp, 

 which has been extensively studied by Emerson and his associates. 

 They find that the mosaic factor frequently becomes simple by 

 mutation either to all red or to all white, and that mutation occurs 

 oftenest to that component of the mosaic which is most extensive 

 in the parent. This result is easily understood if we suppose that 

 the mosaic "gene" divides in such a way as to include more of one 

 of its components than of the other in a cell product, or so as to 

 contain only the major component in a cell product. That cell 

 would then develop into a self-colored individual. I think that 

 this idea of a mosaic gene will explain the high degree of variability 

 which is found not only in striped maize, but also in spotted mam- 

 mals, whether spotted with yellow or with white. Hooded rats and 

 Dutch rabbits are examples of spotted mammals which I have stud- 

 ied extensively and have found to be amenable to selection like the 

 striped pericarp of maize studied by Emerson and by Hays, probably 

 for the same reason, viz, because a mosaic gene varies (or, if you 

 prefer, "mutates") in the proportions of its constituents. 



