Just wondering CS, the only thing about this story that you wondered about was wear was Tevel? The rest you were ok with?
CA- if two-headedness is a dominant gene, then the “guy from Tevel” could have had 1 gene for two-headedness, and 1 for one-headedness. Since he only gives one copy to each kid, each kid has a 50% chance of inheriting the dominant gene and being two-headed. If he had a large number of kids, the two headed variety would be close to 50%. But in a regular size family, it could very well be that only 1 got the gene- just like you sometimes see a family of all/mostly boys or all/mostly girls, even though it should be 50/50. Of course, this might not be a 1-gene trait, or there may be incomplete penetrance, so the laws of inheritance would be more complicated.