Of course it is possible that there are alleles with a large positive effects on intelligence, but is very unlikely because of the complexity of human intelligence.
Some years ago, I read something that suggested otherwise. IIRC, it suggested that we had some idea of what alleles positively influenced intelligence based on the fact that Ashkenazi Jews were something like 2% of the population but had won something like 10% of Nobel Prizes and about 60% of the population had one or more alleles for serious genetic disorders known to impact neurology.
I remember this in part because, although I am not Jewish, I have one of the genetic disorders they tend to carry.