Originally posted by carver9Toriyama stated in an interview that tails are a recessive trait when it comes to Saiyan hybrids -- so basically, they have a 50/50 shot of being born with a tail. Gohan happened to be born with one, but Goten/Trunks weren't. Luck of the draw, I guess.
Why was Gohan born with a tail but Trunks and Goten wasn't?