Methods and compositions are provided for generating F0 fertile XY female animals. The methods and compositions involve making XY pluripotent or totipotent animal cells in vitro cell cultures or embryos that are capable of producing a fertile female XY animal in an F0 generation. Such cells embryos and animals can be made by silencing a region of the Y chromosome. Optionally the cells can also be cultured in feminizing medium such as a low osmolality medium and/or can be modified to decrease the level and/or activity of an Sry protein. Methods and compositions are also provided for silencing a region of the Y chromosome in an XY pluripotent or totipotent animal cell or in vitro cell cultures embryos or animals derived therefrom by maintaining an XY pluripotent or totipotent animal cell in a feminizing medium. Methods and compositions are also provided for maintaining a population of XY pluripotent or totipotent animal cells in a feminizing medium and selecting cells or clones having increased capabilities for producing a fertile female XY animal in an F0 generation. Methods and compositions are also provided for screening for compounds with feminizing activity or for optimizing concentrations of components in feminizing media.
