In the manga and anime, the Saiyan tail pops in and out of the series before disappearing completely with no explanation. Until now.
Full-blooded Saiyans are all born with tails, and because Saiyan genetics and traits are dominant offer a humans, all Saiyan offspring should have tails too, right? Gohan did, so why not the others? The answer is actually kind of interesting. Watch the video below and see for yourself.