Further Studies on the Relationship between the Genotype and the Mammary Tumor Agent in Mice

Abstract

This study investigates genetic control over the propagation and transmission of a mammary tumor agent. Strain C3H female mice with a mammary tumor agent were outcrossed to strain C57BL males without the agent and genetically resistant to it. The female offsprings were backcrossed to strain C57BL males through 7 successive backcross generations. By the 3rd backcross generation the agent has been eliminated by concentration of strain C57BL chromatin; this suggests that strains differed by only a few genes controlling the propagation and transmission of the agent and possibly by only a single pair of genes. Females of the 7th backcross generation were then outcrossed to C3Hf males (without the agent) and their female progeny backcrossed to C3Hf males through 4 backcross generations. With the concentration of C3H chromatin the mammary tumor agent did not reappear suggesting that the concentration of C3H chromatin the mammary tumor agent did not reappear suggesting that the concentration of C57BL chromatin had eliminated the agent rather than causing it to mutate to a latent or inactive form. Mammary tumors observed at an early age in females with the tumor agent were mainly adenocarcinomas of types A and B; some of the tumors occurred in older females with or without the agent. Type C adenocarcinomas and adenoacanthomas were observed mostly in females of an advanced age both with and without the agent. It is possible that this occurrence is associated with histologic changes of the mammary gland with advancing age rather than directly related to the agents absence.