Based on the previous observation that a single major autosomal gene controls susceptibility to Friend leukemia virus in mice, an attempt was made to place the gene for susceptibility, FvS, from susceptible DDD mice into the genetic complement of resistant C57BL/6 mice. The backcross system was adopted for this purpose, the heterozygotes being selected by progeny test at each generation. During successive backcrosses, the effect of gene FvS was not diluted out, and progeny were almost always

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... ained as expected from the single-gene hypothesis, with respect to both genotype and phenotype. With the eighth backcross generation, brother-sister mating was done between the heterozygotes, and it produced mice homozygous for gene Fv8. These susceptible homozygotes and their progeny produced by incross could be assumed congenic with C57BL/6 mice except for susceptibility to Friend leukemia virus. The results indicate that the appearance of early splenomegaly in Friend virus-infected mice is under the control of a single major autosomal gene. transplantation, p. 457-491. In E. L. Green (ed.), Biology of the laborato'y mouse.