In nature, there are different types of conserved patterns of sex-determining mechanisms which range from male heterogamety (XY) found in the majority of mammals and female heterogamety (ZW) found in most birds to environmental sex determination (ESD) expressed most commonly as temperature sex determination (TSD) found in reptiles and some fish species [9].
The independent origins of multiple sex chromosomes via centric fusion involving a cryptic Y chromosome in the widely divergent Megupsilon and Garmanella suggests that cryptic male heterogamety (XY) might have characterized the most-recent common ancestor of the Cyprinodontina.
Gender determination by heterogamety, XY (male heterogamety) or ZW (female heterogamety), is a well-known system involving 2 genetic factors (Yusa 2007).
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