Going on the assumption that it has to do with genetics leads to an interesting conclusion...
If it was because of genetics, the gene for a tail would have to be dominant; Gohan would have one gene for a tail (from Goku) and one not (from Chichi). Since he does have a tail, the tail gene is dominant and the other recessive.
Therefore anyone born with one or two tail genes would have a tail. So Goten would have to have two non-tail genes, one recieved from each parent. This would mean Goku had passed one recessive gene on to him and one dominant gene to Gohan.
This, however, means that full blooded Saiya-jins can carry the recessive non-tail gene; therefore, [B]some Saiya-jins would naturally be born tailless[/B], even if both parents had tails.
Did that make any sense? Sorry, I'm taking AP Biology ^_^'