It has been known for some years that the tendency toward cancerous growths is inherited in mice. Just what the mode of inheritance is has been the subject of much discussion and debate, ranging from the hypothesis that this tendency is transmitted as a simple mendelian recessive (as postulated by Slye) to the theory of Lynch and others that cancer is transmitted as a mendelian dominant and is dependent upon a number of genes for its manif es tation. Much of the food for this discussion arose

more »

... cause of two basic weaknesses in the experimental work which had been done upon the problem: ( 1 ) the fact that the animals used were of not sufficient purity of strain; (2) the tendency of experimenters to combine, in tabulation, all of the types of neoplasia which occurred. Several years ago, two strains of mice were available to the authors which eliminated the first of these weaknesses in technic. One of these, the dilute brown strain, after twenty or more years of inbreeding was developing tumors of the breast in 80 to 90 per cent of the breeding females. The second strain (C57 Black) had not developed a mammary tumor in ten years of inbreeding. It was then decided to cross these two stocks in an attempt to clear up the question of how the tendency toward mammary tumors is inherited. In order to take care of all possibilities, reciprocal crosses were made. That is, dilute brown females were mated to black males and black females were bred to dilute brown males.' According to the mendelian theory these crosses should be identical, and so they proved to be in so far as those alternative characters recognizable in the descendants were concerned. Segregation into the various color patterns, in the F, generations, was exceptionally close to the mathematical expectation. The two crosses behaved very differently, however, in the rates at which they developed tumors of the breast. The percentages were as follows: dBF" 39.82 per cent; dBF" 35.54 per cent; BdF" 6.06 per cent; BdF" 5.96 per cent.d The incidence in the crosses derived from the cancerous mothers (dBF, and dBF,) is thus six or seven times as great as it is in these crosses derived from the non-cancerous mothers (BdF, and BdF?).