According to the Japanese text on the Yoshi trophy in SSBM, there are no genders of Yoshis, they're all the same.
As for how they reproduce, they probably just come from normal eggs, and aren't messed around with (As in, if a Yoshi doesn't throw it, it'd hatch into a baby Yoshi over an extended period of time.
Edited by - Vidgmchtr on 10/12/2005 8:42:42 PM