Transmission of the Mammary Tumor Virus by the GR Mouse Strain. II. Genetic Studies2

Abstract

The genetic basis of congenital transmission of mammary tumor virus (MTV) by GR females was studied with F₁ F₂, and backcross populations of GR X BALB/c (C^-) mice. All mice were foster-nursed by MTV-free C^- mothers. The occurrence of 1 or more of the 3 kinds of mammary lesions (pregnancy-dependent tumors, pregnancy-independent tumors or hyperplastic nodules) in force-bred females within the first year was considered an indication of MTV infection. A mouse without mammary lesions was still considered MTV-infected if she transmitted MTV through her milk. One hundred percent of reciprocal F₁ hybrids developed mammary tumors by 7 months, and the incidence of mammary tumors in (GR × C-) × GR backcrosses was also close to 100%. From the criteria used to determine MTV infection, 75% of the (GR × C^-) × GR backcross females were MTV-infected. Although mammary lesions were detected in only 77% of the F2:[(GR × C-) × (GR × C^-) populations, some lesion-free mice also transmitted MTV by milk. It was predicted that at least 90% of the F₂ females were likely to be infected with MTV. The ratios of MTV-infected females in F₁ F₂, and backcross populations appeared compatible with the hypothesis that 2 independently segregating dominant genes in the GR mouse strain are involved in congenital transmission of its MTV, and that either one of these genes alone is adequate for MTV transmission.