Because as far as color goes, the dominant gene is the black gene and the child will not look white.  It is also because of a bias against blackness by some groups.  If a child has a certain percent of black heritage, they are considered black.  This is not at all unusual nor limited to black and white.  It is a way of preserving culture, hence a child with a Jewish father but a Christian mother is not considered Jewish.  Bias is not always about hating something.  It is about a love of something too.   That Obama was scarred says more about his lack of proper upbringing than about racism.  He should have been taught to love himself and his enemies and to overcome evil with good rather than vengeance.