Genetic diseases are caused when a person inherits an abnormal or mutated gene. If an abnormal gene is "dominant," disease develops even if a person inherits the abnormal gene from only one parent and receives a normal gene from the other parent. If an abnormal gene is "recessive," disease develops only if a person inherits an abnormal gene from both parents. In special cases, genetic disease develops when a person inherits an extra, third copy of a normal gene or inherits only a single normal gene and not the second copy.
Cystic Fibrosis
Cystic fibrosis is caused by a defect in the CFTR gene, which manages the movement of salt and water in and out of the body's cells. The abnormal gene is recessive, so a person must inherit a copy from each parent to develop the disease. According to the National Heart, Lung and Blood Institute, symptoms vary from person to person, but the most common is formation of unusually thick mucus in the respiratory system. This mucus is difficult for the patient to cough up, which leads to recurrent lung infections and eventually to lung damage. Other systems can also be affected, including the digestive tract and the reproductive system.
Huntington's Disease
Huntington's Disease is a progressive degenerative disorder of the brain that first appears between the ages of 30 and 50. Symptoms listed on MedlinePlus, the website of the National Institutes of Health, include progressive loss of muscle control and problems with swallowing, cognition and emotional stability. The HD gene is dominant, so inheriting only a single copy from one parent will cause a person to develop the disease. The average life span after symptoms appear is 15 to 20 years.
Neurofibromatosis
Neurofibromatosis is a genetic disease caused by a dominant gene that stimulates tumors to grow on nerves. According to Genetics Home Reference, a National Library of Medicine website, the more common form of this disease, Type 1, occurs in one of every 3,000 to 4,000 births worldwide.This form is chronic and affects mostly the peripheral nerves outside of the brain and spinal cord. A rarer form of neurofibromatosis, Type 2, can cause tumors on the spinal cord or on the nerves of the ears, resulting in deafness and problems with balance.
Hemophilia
Hemophilia is an inherited disorder that prevents the blood from clotting normally. Abnormal bleeding into joints causes pain and deformity. Sometimes poor clotting also results in bleeding into the brain and other vital organs. Hemophilia is an X-linked disorder, according to the Genetics Home Reference website. Because a boy has only one X chromosome, he will develop the disease if he receives a single copy of the recessive gene from his mother. A girl, however, has two X chromosomes, so she will develop hemophilia only if she receives a copy of the defective gene from both parents. Because of this, hemophilia is much more common in males than in females.
Duchenne Muscular Dystrophy
Duchenne muscular dystrophy causes a person to develop progressive muscle weakness and poor coordination of movement. It can also lead to tightening of muscles around joints, producing significant loss of mobility. Like hemophilia, the disease is caused by a defective gene that is carried on the X chromosome, so it is also more likely to affect males. When a female carries one copy of the defective gene and another. normal gene, she will sometimes show minor muscle weakness or mild heart problems, according to the Genetics Home Reference website.
Polycystic Kidney Disease
Polycystic kidney disease is carried by a dominant defective gene; it may appear in childhood but is more common in adulthood. It causes fluid-filled cysts to form in the kidneys, which may compromise their function and ultimately cause kidney failure. Polycystic kidney disease affects nearly one in 1,000 people in the U.S., according to MedlinePlus. There is a rarer, more serious form of the disease that is produced by a recessive gene. When a baby receives a copy of this gene from each parent, his kidney problems are severe, and the disease usually results in death in infancy or childhood.
Down and Turner Syndromes
Sometimes genetic diseases are caused when a person inherits too many copies of a normal gene. According to the Genetics Home Reference website, the cause of Down syndrome is an error in formation of the egg or sperm that results in a baby with three copies of a gene in each cell instead of the usual two copies, a condition called "trisomy." Babies born with Down syndrome develop slowly and have various degrees of mental disability.
Other genetic diseases can be caused by a missing gene, such as Turner syndrome. The Genetics Home Reference website states that Turner syndrome develops when a female receives only one X chromosome instead of two from her parents. The syndrome causes short stature, malfunctioning ovaries and infertility. Heart defects sometimes develop in these girls as well.
References
- National Heart, Lung and Blood Institute: Causes of Cystic Fibrosis--What are the Symptoms and Signs of Cystic Fibrosis
- MayoClinic.com: Huntington's Disease--Causes
- Genetics Home Reference: Neurofibromatosis Type I Inheritance
- Genetics Home Reference: Hemophilia
- MedlinePlus: Polycystic Kidney Disease
