Many genetic diseases occur because a person is missing the gene for a single enzyme. Here are some of the more common problems caused by missing genes:
- Lactose intolerance - The inability to digest lactose (the sugar in milk) is caused by a missing lactase gene. Without this gene, no lactase is produced by intestinal cells.
- 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 sun tans, hair color and eye color. Without tyrosinase, there is no melanin.
- Cystic fibrosis - In cystic fibrosis, the gene that manufactures the protein called cystic fibrosis transmembrane conductance regulator is damaged. According to Encyclopedia Britannica: The defect (or mutation) found in the gene on chromosome 7 of persons with cystic fibrosis causes the production of a protein that lacks the amino acid phenylalanine. 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. The thick, sticky mucus accumulates in the lungs, plugging the bronchi and making breathing difficult. This results in chronic respiratory infections, often with Staphylococcus aureus or Pseudomonas aeruginosa. Chronic cough, recurrent pneumonia, and the progressive loss of lung function are the major manifestations of lung disease, which is the most common cause of death of persons with cystic fibrosis.
Other genetic diseases include Tay-Sachs disease (damage to the gene for the enzyme hexosaminidase A leads to an accumulation of a chemical in the brain that destroys it), sickle cell anemia (improper coding of the gene that produces hemoglobin), hemophilia (lack of a gene for a blood-clotting factor) and muscular dystrophy (caused by a defective gene on the X chromosome). There are something like 60,000 genes in the human genome, and over 5,000 of them, if damaged or missing, are known to lead to genetic diseases. It is amazing that damage to just one enzyme can lead, in many cases, to life-threatening or disfiguring problems.