A child is born with albinism, an inability to produce or deposit the pigment protein, melanin, in skin cells, as well as other pigment-containing cells of the body. His parents do not display the trait of albinism.
How is this possible?
The child has only one allele for skin pigmentation, though his parents each have two. Having fewer genes produces his albino characteristics.
A spontaneous mutation, not present in either parent, has resulted in the child producing the albino protein. The albino protein displaces the normal melanin.
The normal DNA from both parents has combined to produce a new form of DNA in the child. This abnormal DNA codes for the albino protein.
The parents each have one normal DNA allele and one that codes for albinism. The parents produce enough normal melanin protein to allow normal skin color, but the child cannot.