Background information for your imagination:
4 years before Gohan was born Raditz on his journey to another planet came to check on his brother Goku on planet earth and see what evil things he has done. When he see's Goku hasn't done any harm he got mad, but decided to leave let Goku train just a little more before he encounters him. He needed to take care of his business first. Before he leaves he sees this very beautiful young lady, and takes advantage of her because she was just a weak earthling. (Pretty bad huh, I'm shocked too.) Then he gets on his spaceship and leaves to another planet soon to returning. The young gal, ended up wanting to keep Raditz's son and named him Tar. She raised Tar, and trained him how to defend himself, and taught him as much Martial Arts as she could. Raditz soon comes back, to revisit his brother Goku, and the rest is fulfilled in the story line of Dragon ball Z.