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Show 1903.] IN FANCY MICE AND RATS. 75 was actually made. The first generation were "agoutis " of very large size. Later generations gave amongst others a strain of blue, and of black-eyed white. A strain of agouti has also been saved from it. He tells m e that this formerly had the white feet, a character he carefully bred out. Such a cross may have affected the whole race of fancy mice at the present day. Our search for structural characters referable to sylvaticus, however, has failed to show any case of one pair of pectoral mammae (as in sylvaticus) or any case of long hind foot. All specimens examined were pure musctdus in these features. On the other hand, a feature sometimes seen in fancy mice, and greatly valued by exhibitors, is a large eye, much exceeding the size in an ordinary musculus. But this eye, though large, is still smaller than that of sylvaticus. Nevertheless the large eye is a modern feature in the fancy, and I think it not impossible it may have been derived from a sylvaticus ancestor. Further experiment alone can decide this question. In order to appreciate what follows, the reader must have some acquaintance with at least the outline of the Mendelian principles of heredity. In their simplest expression these principles, as they are exhibited for instance in the experiments of Cuenot (12), are easily comprehended; but when we pass from these simplest phenomena to the more complex facts elsewhere witnessed, we soon reach difficulties which our experimental evidence is as yet only adequate to elucidate tentatively and in part. Cuenot experimented by making reciprocal crosses between albino, pink-eyed, fancy mice, and wild grey mice {21. musculus). H e was careful to use wild mice in order to be sure that his coloured form was pure. As a result he obtained always and without exception grey mice. In Mendelian terms, grey is therefore dominant over albinism, which is called by contrast recessive. The first filial generation thus produced, which we may conveniently call Fx, when bred inter se, gave a total of 198 greys and 72 albinos, constituting the second filial generation, or F2. The ratio of dominants (D) to recessives (R) is here 2"75 to 1, a fairly near approach to the ratio 3:1, which on the simplest form of the Mendelian hypothesis is to be expected. In other words, the facts are, as Cuenot stated *, in agreement with the supposition that in the formation of the gametes of the hvbrid Fx, there is complete segregation of the grey colour from albinism, and that in both male and female hybrids there are on an average equal numbers of gametes produced bearing each of these two characters. According to the same hypothesis, the grey mice in F2 should consist of pure or homozygous greys (DD) and of heterozygous o-reys ( D R ) in the proportion of 1 : 2. Cuenot tested this to some extent by breeding the F, dominants * Cueuot's paper seems to be the earliest application of Mendelian principles to animals. |
