Its complex and involves gene flow and the distribution of alleles across distinct populations versus within distinct populations. Basically, there is more variation within a population than between, and no metric, or stereotype if you will, which could be used to define a distinct population based solely on their phenotypic expression is able to be applied equally to all members within that population. So for instance, with dog breeds, you know a rottweiler looks like a rottweilier, meaning it bears the standard markings a rottweilier would bear, its within the size range for a rottweilier, its pedigree states that it is such, etc. None of this exists for humans. Within a population of any particular group, you will find elites who are smarter, you will find plebes who are less intelligent. Blacks in Africa do have complex cultures but the climate variations made it difficult for them (as well as Native Americans) to successfully build large cities which could be supported through agriculture and the like. Asians are smaller generally due to lower nutrition because rice lacked (until golden rice) key nutrients required for growth, such as Vitamin D. Further, Asian countries were actually plundered and looted as soon as it became viable for someone to do so. China, Korea and others had a geographic advantage in the Himilayas/Hindu Kush Mountain ranges and the Gobi Desert, both features which helped separate most of Asia proper from the West, just as the Atlantic kept the Native Americans free of outside influence, until technological barriers to travel over long distances were resolved.
Political correctness necessitates that positive stereotypes are acceptable fodder for comedy (big black dicks and great at sports would be an example of that, anyone who watches enough porn knows there's plenty of white dudes with retarded big dicks too for instance, and there's been plenty of white super athletes over the years, such as Larry Bird).
Mostly the reason why blacks and hispanics underperform is because there is zip, zero, nada influence or involvement by the parents at home in their educations, for all sorts of reasons. In the case of hispanics who are the children of 1st generation immigrants, often the parents can't speak English and don't have any education of their own to speak of, and in the case of African Americans, its because they've been taught that getting a decent education is basically "Being white".
It really has little to nothing to do, overall, with genome. The contribution to intelligence that specific genes provide has been tagged at a very low percentile, something like 2%. Intelligence is more about how someone is raised than just flat natural talent as a result of luck by the combination of genes which contribute to intellect. For instance, Einstein's brain has been examined, and its been shown that the region of his brain which allowed for things like spatial cognition and mathematical computation was larger than the average person's. This is not because of some mutation in his genome that led to that portion of his brain being more developed. Its because he was USING that part of his brain constantly during formative years, slowly gearing it towards a specific purpose. Same thing with people who speak many languages. The portion of the brain which allows for speak recognition and the like is larger from constant utilization.
Yes, genome plays some small part, but the idea that biology is destiny is flawed, profoundly. Biology interacts with environment and culture to create an end product, an individual.
added: Also, a distinction has to be made between the heritability of genes which potentially influence intelligence, and the actual degree to which any one or a combination of those genes influences intellect.
Genes give you a range of potential from low to high, but the degree to which you achieve that potential is implicit on a number of other factors. Another example of this is height. Stature is highly heritable, yet when people from very poor countries or areas with low nutrition move to somewhere where nutrition is much higher, the subsequent generations of children they have, provided they have access to the better nutrition available in the new area, grow taller than their parents, and the taller still than their grandparents. Genes give a range, but environment and culture determine where you finally land in that range.
