In Utero Transmission of the Mouse Mammary Tumor Agent

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Abstract
Transmission of the mammary tumor agent of the male parent was studied by mating 25 agent-free strain BALB/c female mice to agent-carrying strain C3H males to procure F1 hybrids. The F1 hybrid females were bred to agent-free strain C3H males to obtain first-generation backcross mice and these females were also bred to agent-free C3H males to supply second backcross progeny. Thus, 2 backcross generations served as “progeny test” mice for the presence of the agent in their female F1 hybrid or BALB/c ancestor. The C3H father's agent gained access to the descendants of 6 BALB/c females. In 4 families the father's agent infected the BALB/c female because thereafter all F1 hybrids and their backcross progeny developed mammary tumors. In the fifth family 2 F1 hybrids of the third litter and their backcross descendants carried the agent, whereas an F1 littermate and her descendants did not. An F1 hybrid of the fourth litter and her descendants were also free from the agent. In the sixth family all 4 F1 hybrids of the third litter and their descendants were infected, but in the fourth litter only 1 of 3 F1 hybrids was infected. A hybrid of the fifth litter was not infected. The last 2 families suggest in utero infection of the F1 hybrids without concomitant infection of their BALB/c mother.