Re: Question about twins
Fraternal twins run in families; identical twins don't. The current theory for how the "twin gene" works is that it encourages multiple ovulation -- both ovaries ovulating at the same time -- so that multiple fertilization is more common. Of course, multiple ovulation can happen even without the gene.
It is passed through both sides of the family -- while the boys themselves won't be more likely to have twins, their daughters will, because they can pass the gene on. So yes, if your great-grandfather's family had the twin gene, it could have passed through to you, whether he was on your mother's or on your father's side. Given the small sizes of American families, it could well not have manifested itself in several generations. On the other hand, there might have been other reasons your great-great-grandmother ovulated multiple times, or that particular gene might not have been passed to your great-grandfather, or even if he had it it might not have passed those next two generations to you. So you never know.
Usually with twins the doctor does hear two heartbeats at around the stage you are at now. However, often enough he does not, and it is a surprise at the ultrasound. And in rarer cases, the ultrasound also fails to pick up the second baby, usually because it's hiding behind the first, and the surprise comes later on, either at birth, or earlier when they notice that your uterus is growing too fast and check again.