Yes, there are genetic differences between people. And two dark humans are more likely to have a dark child than two light humans. But there are enormously bigger differences noted within a race than any of the differences between two races.
The reason "racial" differences are so noted is because they're visible. The incredibly few genes that wind up being very visible happen to be noticed. Many people presume that this means that the rest of the genes are different too. Not the case.
The human species happens to be one of the most homogeneous, because of our historical tendency to travel. All the genes got everywhere. It's just that the light skinned genes never survived in tropical areas... the sickle cell anemia gene never survived anywhere where malaria wasn't a problem.
At the root of the matter, yes there are differences between people, but no racial classification can be scientifically approved. You say a Spaniard is different from a Moroccan, but people have traveled back and forth ever since the times the first hominids crossed the straight between them. The genetic difference between two Spaniards (who are not relatives) is just as great as the difference between a Moroccan and either of the two Spaniards.
It's not as if Africans and Europeans were ever at any point isolated from one another. People always traveled. So did genes.
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