The effect of continued inbreeding on the tumor rate in mice

Abstract

In former investigations 1 we found that different strains of mice differ very much in their tumor rate and tumor age; and that these differences in tumor rate and tumor age were maintained and hereditarily transmitted to the following generations throughout the period of observation, notwithstanding the similarity of external conditions under which the animals were kept.

It was of interest to inquire how continued inbreeding of these various strains would affect the tumor rate and tumor age.

We found that while in the majority of strains the differences in tumor rate and tumor age remained constant in the successive generations, in others certain variations became noticeable as a result of the continued propagation of these strains. In a few cases an increase in the tumor rate occurred in the later generations, but in the large majority of cases in which a change was observed there was a tendency towards a decrease in the tumor rate which in some instances was very marked.

These changes in the tumor rate concomitant with continued inbreeding seem to depend on the following two factors: (I) As a result of long-con tinued in breeding in mice certain characteristics of a strain may change, the strain becoming less prolific and less vigorous. These changes may be accompanied by a lowering in the tumor rate. This was especially evident in the case of the “ No. 8 “ strain which had been inbred through the largest number of generations and through a considerable number of years. (2) In other cases it could be shown that in the course of continued propagation certain families, which were more resistant to certain diseases or which were naturally more prolific or otherwise favored by accidental conditions began numerically to preponderate in later generations. In some cases such families or substrains differed in their tumor rate from the main strain and thus as a result of selection within a strain differences in the tumor rate appeared in the course of continued inbreeding.

Our further investigations confirmed and still further emphasized our previous conclusion that on the whole in the mice with a higher tumor rate the tumors appear at a relatively earlier age than in those strains in which the tumor rate is lower.