The answer is gene mutation.
Albinism is a congenital disorder characterized by a complete or partial absent of pigment in skin, eyes or hair. It is a recessive disorder, which means that parents must not have the disorder. But if both of them carry a recessive allele, and offspring inherit the recessive alleles from mother and father, the offspring will have albinism. Anyway, it is pretty rare, since it is the recessive trait, and most of the offspring will have normal coloring.
