Calico cats vs. Chimeric mi

[douglas gray cancer research]

When XY ES cells end up in an XX blastocyst, the resulting mouse is
a fertile male that will transmit the ES genome (although at variable
efficiency).  The explanation is that androgenic hormones are secreted
by cells of ES origin, thereby "converting" the mouse into a phenotypic
male (even though XX cells are still present).  This is precisely why
male ES cells are used - there is the bonus of converted XX blastocysts,
and it is easier to establish a breeding colony with a male than a
female.  Hope this helps.

~r dg