Directional dominance and a developmental model for the expression of the Tda testis-determining autosomal trait of the mouse.

The POSCH-2 Y chromosome from the poschiavinus variety of Mus musculus domesticus causes incomplete testis development in the recessive autosomal background of the C57BL/6J laboratory mouse strain. Testis development is normal with the POSCH-2 Y in its native strain background as well as in some strains of the laboratory mouse such as DBA/2J. The phenotype or expression of XY gonadal hermaphroditism in a C57BL/6J strain, which was constructed to be consomic for the POSCH-2 Y, is a threshold trait in which liability is normally distributed and thresholds in the development of the testis define the probability of observing XY embryos with different combinations of ovaries, ovotestes, and testes. The difference in this testis-determining autosomal or Tda trait between the C57BL/6J and DBA/2J strain pair has been demonstrated to be multigenic. We conducted a survey among different strains of the laboratory mouse by test mating females with C57BL/6J.Y-POS males that are consomic for the POSCH-2 Y. We identified five groups of strains with significantly different response of XY gonadal hermaphroditism in their XY-POS F1 test embryos. In test embryos, four groups of strains produced gonadal hermaphroditism with different distributions of the types of gonad that appear to have the same variance or shape of a normally distributed liability, but the means of the distributions are at different locations on a scale of gonadal development. The fifth group of strains produced only tests in the test embryos. Several additional matings produced results suggesting that a model of dominance, in the direction of more complete testis development, could interpret the strain differences. The differences in response to the POSCH-2 Y chromosome among the five groups of strains may represent the phenotypes of the genetic recombinants in the Tda trait that were suggested previously by a segregation analysis between C57BL/6J and DBA/2J. The strains may also provide the tools to further dissect the allelic differences and locus determinants of the Tda trait.