Abstract
Mice with six legs appeared about two years ago in a stock which had descended from five individuals and had been inbred for several generations. Since this stock had been subjected to no special treatment, the possibility of the anomaly having a purely hereditary basis suggested itself, and in the fall experiments were undertaken to determine if such were the case.
In the course of the tests many anomalous individuals have been produced ranging from Y-shaped specimens with four hind legs and two tails, to those with a relatively slight degree of doubling in the external genitalia. These may all be referred to as “doubles.” Apparently connected with the same manifestation is the appearance of hemorrhagic testes, spina bifida, and occasional other anomalies such as microphthalmia. The anomalous individuals are for the most part incapacitated for breeding, so their parents and sibs have been isolated from the rest of the stock and used for the experiments.
At first anomalous individuals were produced about equally by the selected (D) and unrelated (B) strains, but gradually the former began to show an increased incidence which has risen to about 12 per cent (113 :805) for the whole A strain and about 20 per cent (45:178) for the best producers. In the meantime doubling has been practically eliminated from the B strain, only two anomalous individuals having appeared among the last 1100 or more young. In a few instances the character of the anomaly has been such as to permit of the production of young, and one mating between two anomalous individuals has been successful. Results of different types of matings are as follows :
In view of the fact that the animals are all closely related, have been kept in the same room, and received similar treatment, there can be little doubt that the anomaly has a germinal basis which, through selection, has been mostly segregated in the D strain and practically eliminated from the B strain.
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