Many genetic diseases occur because a person is missing the gene for a single enzyme.
Albinism - In albinos, the gene for the enzyme tyrosinase is missing. This enzyme is necessary for the production of melanin, the pigment that leads to suntans, hair coloring and eye color. Without tyrosinase there is no melanin. In cystic fibrosis the gene that manufactures the protein called cystic fibrosis transmembrane conductance regulator is damaged. This flawed protein somehow distorts the movement of salt and water across the membranes that line the lungs and gut, resulting in dehydration of the mucus that normally coats these surfaces.