Animals from different genera can hybridize if the two genera aren't too distantly related. In fact, genera are rather arbitrary groupings in many cases, not defined by reproductive compatibility but rather subjective ideas about "similarity".

That includes turtles. See the cool examples below:

Map turtle x Red-Eared Slider
http://www.theturtlesource.com/turtle_inventory.asp?cat=333&id=100200405

Blanding's x Wood Turtle
http://www.woodturtle.com/hybrid.html