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Show 1903.] IN FANCY MICE AND RATS. 87 some of the coloured individuals contained no albino. As far as the few observations went, the extracted albinos gave only albinos. So far therefore the Mendelian hypothesis harmonises well with the phenomena. When, however, we begin to consider the relations of the several colour-types to each other, we meet some important problems. The original waltzers are described as black-and-white. Of what pigments the black was made up we do not know. Probably it contained both the black and the brown elements. However this may have been, the reversionary heterozygote clearly did, though whether it also contained the yellow pigment is not so clear. On examining the details as to the offspring of the several pairs, it appeal's that though the self-greys may, as the first cross pi-oves, contain all the other 3 coloured types and the albino, the grey-and-white contain the albino only. Similarly the black-and-white can only contain the albino, so far as the evidence goes. But black seems to be dominant over black-and-white. The facts are not sufficient to make these deductions quite certain ; and, in particular, since the evidence in rats shows that grey-and-white may dominate over black-and-white, it may be merely from accident in the choice of individuals that no black-and-white was produced by any of the grey-and-white mice. The families from the 4th pair on Table I., and from the 3rd pair on Table II., are especially interesting as giving indications as to the gametic ratios in a complex case, though the evidence is insufficient to determine these ratios. In the first case black-and- white x black gave 10 black, 15 black-and-white, 7 albino. Both parents were heterozygotes containing albino, being each raised from self-greys x white. From the facts it is clear that one parent at least was giving off gametes black, black-and-white, and white; and from the indication that black is dominant to black-and- white, it is probable that this parent was the black. The simplest supposition is, then, that the black-and-white gave off blw. and w., and that the black gave off bl. and blw. in equal numbers, and whites equal to then sum. This distribution would give the ratio 1 bl.: 2 blw.: 1 w., and where experiment gave 10 bl.: 15 blw. :7 w. we should expect 8 bl.: 16 blw. : 8 w., which fits well. But in the 3rd pair on Table II. we have a blk. X albino giving 7 bl.: 16 blw.: 20 w., where, on the hypothesis suggested, w e should expect equality between bl. and bl.w., and the discrepancy is considerable and emphatic. Pending further experiment, the relations of bl. to blw. and to white cannot be stated with any confidence. Another point |
