According to a comment in this very interesting video about how to get large classes engaged in active learning, males are more likely to put their hand up and to be called on in class. Best practice is therefore to do random cold calls. Students hate that, and so I compromise: for extra credit, they can have their names in a hat from which I randomly select people.
The class is 44% male. 40% of those who opted to have their names in a hat were male. So no bias there. The real bias is probably in the personality types selected. That sort of bias might be much more insidious.