OP, intelligence is not inherited, but rather, nurtured. If you have a couple that are of higher intelligence, it's natural to think that they will raise children that have higher intelligence, simply because they're surrounded by it from day one. On the other hand, dimwits would then tend to raise more dimwits, right?
So why don't we have a few dozen Einsteins running around by now? He had two sons and a daughter, it would be natural to assume they each would have at least two children, those offspring would have at least a pair each, etc. Einstein's youngest child was born in 1910, so we can assume at least 4, perhaps 5 generations since then. That would amount to anywhere from approximately 90 to 150 children in this generation. Yet, we hear nothing about the physicists of the world having a dozen Einsteins, coming up with brilliant theories every year, do we?
What you've stumbled upon is more along the lines that ignorance tends to breed ignorance. Probably why we still have racism in this world, even though it's been proven that we're all the same on the inside, with few exceptions (blood type, etc). That's more of a human failing than anything that's passed on through the DNA.
That being said, I suggest you go rent/find/borrow/download a movie called Idiocracy, starring Luke Wilson. Looks as though you're not the only person to wonder about such a thing!! :hmm:
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