Sex determination in mammals is highly conserved, with males being 
heterogamous (XY) and females being homogamous (XX).  However, several 
rodent species exhibit aberrant sex determination in which some females 
have an XY karyotype.  Among these species, members of the genus Akodon 
are 
especially unique because XY females appear to be the consequence of a 
disfunctional Y chromosome (rather than a mutant X).  Maintenance of such 
XY females in natural populations presents a substantial evolutionary 
puzzle because simple population genetics models predict that such a 
chromosome would be lost from a population within only a few generations. 
 We will present a series of simple population genetics models in which 
the 
potential influence of different evolutionary forces are assayed.  The 
results of these simulations will be used to prioritize likely 
mechanisms, 
and to construct hypotheses for more focussed field-based studies.