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 poschiavinus variety of Mus musculus domesticus was introduced into C57BL/6J by backcross matings. The F-1 generation expressed normal testis development in XY individuals with the poschiavinus Y chromosome. In the backcross and subsequent crosses to C57BL/6J females, XY individuals expressed ovaries bilaterally or various combinations of an ovotestis with a contralateral ovary or testis or bilateral ovotestes and a few had 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 state, caused incomplete testis development with the poschiavinus Y chromosome. Subsequent attempts to map the Tda-1 factor, using a recombinant inbred strain approach, failed to localize Tda-1 and this suggests it might map to different chromosomes depending on which strain pairs are used. We constructed two strains of C57BL/6J and DBA/2J that are congenic for the poschiavinus Y chromsome. In the C57BL/6J.Y-POS congenic strain, liability to express incomplete testis development is normally 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 the DBA/2J.Y-POS congenic strain. With the two congenic strains and their 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.