Tannipu's creator gave her a loving and affectionate relationship with Bardock, resulting in motherhood of BOTH Goku and Raditz. Okay, personally, I feel that Goku and Raditz are more like half-brothers than anything else mainly because I'd hate to think of Goku's mother being so ugly since Raditz must've been one hairy baby and looks nothing like Bardock.
Bardock: *makes quite a lewd pass at Sharotto*
Sharotto: *laughs* "Let's just see if you're worth the 5 minutes of my precious time..." ::Checks scouter... It reads 8,000::
Sharotto: *dumbfounded* "Uhh..."
Bardock: *smirks* "Well?"
Sharotto: "... Yes?"
Bardock was incredibly strong for a low-class and Goku's mother was most likely just as strong, possibly a little less. But it could be that the two copulated through the means of, yes, physical attraction and fun, but more importantly comparable power levels in hopes to create an equally powerful offspring, if not more powerful than the parents themselves.
Tannipu was f**king SOFT! Saiyans are NOT doting parents. Bardock didn't even show any consideration for his kid (again, until the last minute). My idea of Goku's mother, Sharotto, is one hard-ass Saiyan woman. So much that she continued to train and battle during pregnancy! Actually, it was a blow that triggered labor. She felt it was only necessary to develop an inborn endurance for her child. And going by infant Goku's constant crying and whining that several characters on more than one occasion have commented to be a "true warrior's cry," I'd say she had the right idea. Tannipu wouldn't have thought of that.