W. E. Castle

Green's Studies of Linkage in Size Inheritance

DR. C. V. GREEN has published in a recent number of this journal (Nov.-Dec., 1931) an important paper dealing with the subject of size inheritance in mammals. His observations were made on a cross between two species or mice which differ in body size but produce fertile hybrids. The larger parent was a laboratory race of the house mouse, Mus nusculus; the smaller parent was a race of Mus bactrianus from China. The large parent was homozygous for three recessive color mutations, nonagouti (a), brown (b) and dilution (d). The small parent bore corresponding dominant allelomorphs, agouti (AwV), black (B), and intensity (D). The F1 animals were heterozygous for all three color genes and were back-crossed with the large triple recessive race for the purpose of detecting possible linkage between one or more of the color genes and the larger size of the recessive parent. Such evidence was obtained. Dr. Greens paper shows careful observation and conservative deduction of conclusions. I am fully convinced of his accuracy in both regards, so far as the hypothesis which he set out to test is concerned. But there are certain considerations which seem not to have occurred to him that conceivably qualify those conclusions. These I wish briefly to present. Greens observations show unmistakable association of larger body size and brown coat color in the back-cross generation. This is shown in the significantly greater adult body weight of both sexes and in the several bone measurements and body length, which are in general greater. To some extent but less emphatically, the dilute individuals are larger than the intense ones. Green does not claim (in this paper, though he did in an earlier one) that larger size is a characteristic of the groups of individuals which have the third recessive character introduced from the large race, namely non-agouti (a). His tables show the adult body weight to be greater in both sexes in the nonagouti animals, but the bone measurements favor the agouti animals. The evidence is thus conflicting. I am inclined to attach greater importance to the body weights than to any other observations made by Green because they alone