Some Mormons believe the GOD cursed Cain with black skin and a flat nose. However, the "mark" was set upon Cain before the flood. In that flood all flesh perished except for Noah, his wife, his three sons, and their wives. If the curse upon Cain was dark skin, the only way the race could have survived was for Noah to be a direct descendent of Cain. However, Noah's genealogy didn't come from Cain, but from Seth. This is all clearly stated in Genesis 5:3-32.
So where do all the races come from? If we are all descendents of Adam and Eve, shouldn't there only be one race? Well, what is race? Reuters news service ran the following article that was written by Maggie Fox:
So lets look at the question again, What is race? According to a recent Webster's Dictionary, race, as it applies to mankind, is
So if enough of the same people from the same area have children who then continue to have offspring with those same people from that same area, they will have similar traits. That sounds like a large scale version of what is typically said at family gatherings - how so and so looks so much like their parents, uncles, aunts, grandparents, etc.