Alma 3:9
Genetically it would be called the dominant gene that when dark and light skinned people mix the children almost always come out dark. We know that it is because when covenant people are led away by those who are raised under false and wicked traditions their children will also be raised with false and wicked traditions. We know that during those times when the Lamanites were righteous their skin often became white like unto their Nephite brethren unless they needed to remain seperate because of the Nephite wickedness.