The genetic basis of XX-XY differences present before gonadal sex differentiation in the mouse.

There is now a substantial body of data showing that in eutherian mammals (mouse, rat, cow and man) XY conceptuses are developmentally more advanced (and consequently larger) than XX conceptuses of equivalent gestational age. This developmental difference is already discernible in the preimplantation period and it has been suggested that the more advanced development of XY embryos may be a consequence of the preimplantation expression of Y chromosomal genes such as Sry or Zfy. In the present paper sex-chromosomally variant mice were used to analyse the genetic basis of XX-XY differences as manifest at 10.5 days post coitum. The results show that the XX-XY difference is due to a combination of a Y chromosome effect and an effect of the difference in X chromosome constitution (2X v 1X). The Y effect is not dependent on the presence of Sry. In the light of this and other studies, it is concluded that the Y chromosome of most mouse strains carries a factor which accelerates preimplantation development and that the resulting developmental advantage is carried over into the postimplantation period. The retarding effect of two X chromosomes is then superimposed on this Y effect subsequent to the blastocyst stage but prior to 9.5 days post coitum.