THE TESTIS-DETERMINING AUTOSOMAL TRAIT, TDA-1, OF C57BL 6J IS DETERMINED BY MORE THAN A SINGLE AUTOSOMAL GENE WHEN COMPARED WITH DBA/2J MICE/

The putative Tda-1 or testis-determining autosomal trait of the C57BL/ 6J mouse strain came to attention when the Y chromosome from the posch iavinus variety of Mus musculus domesticus was introduced into C57BL/6 J by backcross matings. The F-1 generation expressed normal testis dev elopment in XY individuals with the poschiavinus Y chromosome. In the backcross and subsequent crosses to C57BL/6J females, XY individuals e xpressed ovaries bilaterally or various combinations of an ovotestis w ith a contralateral ovary or testis or bilateral ovotestes and a few h ad testes bilaterally. Some of the previous breeding data appeared to support the hypothesis that C57BL/6J had an autosomal recessive factor that differed from the poschiavinus strain and, in the homozygous sta te, caused incomplete testis development with the poschiavinus Y chrom osome. Subsequent attempts to map the Tda-1 factor, using a recombinan t inbred strain approach, failed to localize Tda-1 and this suggests i t might map to different chromosomes depending on which strain pairs a re used. We constructed two strains of C57BL/6J and DBA/2J that are co ngenic for the poschiavinus Y chromsome. In the C57BL/6J.Y-POS congeni c strain, liability to express incomplete testis development is normal ly distributed and thresholds in development specify the probability ( or areas under the normal distribution) of different classes of ovary, ovotestis, and testis combinations. Testis development is normal in t he DBA/2J.Y-POS congenic strain. With the two congenic strains and the ir normal parental strains we were able to conduct standard crosses to examine the reciprocal F-1 and four types of backcross generations to the C57BL/6J strain in which all XY individuals have the poschiavinus Y chromosome. The Tda-1 trait of C57BL/6J is recessive to DBA/2J, but the segregating backcross generations reject the single gene model.