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Show 1903.] IN FANCY MICE AND RATS. 97 the cross with wild grey gave grey Fv (2) Albinos extracted from the cross with black gave black Fv (3) Albinos extracted from a cross with yellows (of complex origin) gave a mixture in F1? either of yellows and greys, or of yellows and blacks. There is therefore a proof that individual albinos, though outwardly alike, m ay belong to several distinct classes, exhibiting different properties in their heterozygous unions {cf Parsons' case, p, 84). The resemblance of the heterozygote to the coloured type from which the albino was extracted is a new fact, the significance of which we cannot yet fully appreciate *. Cuenot tentatively makes the attractive suggestion that the particular colour of the hetero • zygote may depend on the association in the same zygote of various colour-constituents ; and that though the albino is white in itself, it may carry on such constituents from a previous coloured parent. Then, according as one or other of these complementary constituents is brought in by the albino, the heterozygote will show the corresponding colour. The chief obstacle to this view is the fact that when a heterozygote shows reversion (as opposed to simple dominance) the reversion frequently includes various qualities, such as size, temperament, habit of growth, &c, as well as colour. A cognate problem was alluded to (p. 88) in the discussion of von Guaita's facts. His original black-and-white x albino gave a reversionary heterozygote ; yet in F2 both the black-and-white and the grey-and-white D R s present those colours as simple dominants over albino, as their offspring proved. Since in this case no new strain was introduced, the reference to pedigree is not sufficient to elucidate the whole difficulty. The relation of the several classes of albinos to each other seems to be the next point for investigation, and a useful experiment might be made by breeding albinos extracted from one colour, with albinos extracted from another colour, the offspring to be then tested with a single pure coloured race. It is not impossible that the various types of albino will then themselves exhibit phenomena of segregation. The new report of Mr. Darbishire (28) and Professor Weldon's comment (29) have appeared too late for adequate discussion here. It may, however, be remarked that both authors scarcely appreciate the Mendelian view when they state that according to it all albinos (or other recessives) may be treated as " in every respect similar." ISTo one, I imagine, would suppose that the similarity need extend to characters other than the albinism. W e are familiar with cases in which recessives, though alike in the recessive character, are dissimilar in other respects ; and (as stated supra p. 76) may, when crossed together, even produce heterozygotes exhibiting a character known to be dominant over the particular recessive concerned. W e should no more suppose all albino mice to be identical because they were albino, than all white sweet-peas * Possibly it is to this phenomenon that Crampe refers in the statement discussed on p. 79 of this paper. I cannot, however, find a case of Crampe's exhibiting Cuenot's phenomenon. PROC. ZOOL. Soc.-1903. VOL. II. No. VII. 7 |