Is race really a cultural construct?

Here is how one reader at Andrew Sullivan's blog puts it:

If there is no "race" at the genetic level, then why is it that dark skinned people with very curly hair keep having children with dark skin and curly hair? Why do children of Asians look persistently different from children of Jews? There is something genetic happening that affects skin, hair, facial features, disease susceptibility, and a whole host of other things. But we are powerless to examine those differences because there's no such thing as "race" at the genetic level? The contention is absurd on its face.