Heterosis, hybrid vigor, is an increase in function in a hybrid offspring. This effect is responsible for the greatest practical successes of genetics. It has greatly increased the yield of maize, and, more recently, that of rice. It’s also important in livestock breeding.
Probably the the most famous example is the mule, the offspring of a male donkey and a female horse. Mules are hardier than horses, have more endurance, and are stronger. They’re smarter than horses or donkeys. They’re extremely useful in preindustrial agriculture- sterility is their only real disadvantage.
There are three main genetic explanations for heterosis. One idea, the dominance hypothesis, is that undesirable recessive alleles from one parent are masked by dominant alleles from the other parent. The overdominance hypothesis states that two different alleles can have a positive effect that is greater than either alone. The epistatic theory attributes heterosis to positive interactions between non-allelic genes.
Each of these patterns may occur in some cases, but on the whole it looks as if the dominance theory explains most of what we see. Note that this is equivalent to a reduction in genetic load – and thus the success of hybrids might give us a general idea of how much advantage a zero-load individual might have.
Hybrid vigor is not something that always happens when you cross two fairly different parental strains. Sometimes you see reduced vigor (outbreeding depression). But who wants that? When you see a pronounced positive effect, the two parental lines are said to ‘nick’. In general, this happens when the parental strains are genetically fairly distant. But not too distant, of course. Note that this is a first-generation effect.
This suggests that crosses between particular human populations might produce superkids. Clearly, there is no dramatic positive effect in any of the common crosses – we would have noticed. I’ve seen a study that observed a moderate IQ advantage in Japanese-European kids in Hawaii, something like a quarter of a standard deviation, but I don’t think it’s been replicated, and that’s not enough change to get excited about in any event.
However.. there are crosses that have never been studied, quite likely because they’ve never occurred. For example, the two human populations separated by the greatest genetic distance are probably Bushmen and Melanesians/Australian Aborigines: I’ve never heard of a cross and I doubt there has ever been one.
There’s no telling what would happen.