Tiger is not completely of African-American descent. Earl Woods, his father, is half black, one-quarter American Indian and one-quarter Chinese. Kultida is half Thai, one-quarter Chinese and one-quarter white. Are you sure his skin colour is darker than either parent?

Part of the answer is probably that he is outdoors in the sun more. But there is some genetics at work here, which I will explain simplistically. There is more than one gene for "darkness" Suppose there are 10 gene positions where one variant makes the bearer darker then the other (or, to say exactly the same thing, one makes the bearer lighter than the other). And suppose also that they all have exactly the same effect. (Neither of these is true, but they will do to argue). Suppose his parents each have five of the dark version and five of the light. This makes them both about mid-colored. If his parents came from the same community, these would be the same five, so that he would be the same mid-color as his parents. But his parents (and grandparents) come from widely separated communities, which means that they are likely to have different versions of the lightening/darkening genes. In fact, suppose that they have only one darkening and one lightening variant in common, and they differ at all eight other gene sites, each haveing four lightening and four darkening, but the opposites. In that case, Tiger could inherit form his parents anythign from nine darkening gene variants (plus the on lightening one his parents have in common) to nine lightening variants (plus the one darkening one his parents have in common). Which would make him anythign from very dark indeed, to very light indeed. So it is not surprising if some from a geographically mixed ancezstry has a skin colour significantly different from either parent, and not necessarily in between (though in between is still the most likely).