Evidence has previously been submitted in favor of the theory that susceptibility to spontaneous tumors of various types is inherited. The question arose whether susceptibility to tumors induced by tar could be shown to be hereditary by experimenting with the same strains of mice which had already been shown to differ significantly in respect to their spontaneous tumor rates.

Two strains of mice were selected for observation. One strain, the Bagg albinos, has a low rate of mammary gland tumors but a higher rate of spontaneous lung tumors. The other strain, No. 1194, agouti, has a higher rate of mammary gland tumors but a lower rate of tumors of the lung. A previous test showed no difference in the percentages of skin tumors which arose after tar painting.

It has already been shown that the difference in the lung tumor rates is mathematically significant. When the two stocks are treated with tar by applying the irritant, not in the same spot, but on different areas successively, the percentage of lung tumors is increased in each stock, Text-fig. 11. The rate of the Bagg albinos increased from 37.04 to 85 per cent, that of Strain 1194 from 6.73 to 22 per cent. But a difference between them is still maintained, and this difference also is significant mathematically.

When the two strains are crossed and the offspring subjected to the tar treatment, the latter give a high percentage of lung tumors—79 per cent in 28 individuals—about the same as the parental high tumor strain. When the F1 sons are backcrossed to the original

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stocks, the cross to the high tumor strain maintains the high tumor rate, 81 per cent in 37 mice, while in the cross to the low tumor strain the percentage drops to 39 per cent in 38 mice. This result indicates that susceptibility to tar-induced tumors in the lung is hereditary. The number of factors concerned has not yet been ascertained. Possibly one or more of them is dominant.

In general, the conception that susceptibility to pulmonary tumors is hereditary seems to be upheld by the fact that the two strains of mice described differ conspicuously in respect to spontaneous tumor rates under ordinary laboratory conditions; the strains differ also under experimental conditions, as described in this report; and when crossed, their offspring by suitable backcrosses, will again show significant differences.

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