A father is required to redeem his firstborn son by paying some money to a kohen. A mother is not. The Bavli, Kidushin 29 amud 1, asks, "How do we know she's not commanded?" and answers:
[In the Torah] is written "תפדה / you shall redeem", [which can be read] "תפדה / you shall be redeemed", [and we connect the two readings:] whoever is commanded to redeem himself [if no one else did] is commanded to redeem others, and whoever is not commanded to redeem himself is not commanded to redeem others.
My kid asked: That's patently false! A second-born is not commanded to redeem himself, but is still commanded to redeem his own son. [I might add that the same is true of the maternal grandson of a kohen, and of anyone born by cesarean section.] That doesn't seem to be a class of person less worthy of consideration than females. What does the Bavli mean here?