No, thankfully there is no gene for being dumb! And so far, we understand very, very little of any potential gene for intelligence. Many studies have shown that intelligence tests are biased towards more western cultures (USA, Canada, Western Europe), and towards males. Add in all of the differences we have found in how people of different races, cultures and sexes are treated through their lives, and the cultural reasons for intelligence are gigantic!
So, one day we might pull away all the layers of culture and see what there might be in the genes. But I would not trust much research in this direction if it doesn’t somehow control for culture.