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defective gene. If two carriers of a particular recessive disease gene have children, each child has a 50 percent chance of receiving a healthy copy of the gene from one parent and a defective copy from the other parent, and of being a carrier like his or her parents. Carriers can pass on a defective copy of a recessive gene through several generations without any of the children having the disorder. Each child of two carriers has a 25 percent chance of receiving two defective copies of the gene and having the disorder, and a 25 percent chance of receiving two healthy copies of the gene and being unaffected.

Cystic Fibrosis

Cystic fibrosis is an autosomal recessive genetic disorder that causes persistent lung and digestive problems. The genetic defect causes the lining of the airways leading to the lungs to produce excess mucus that clogs the lungs and makes the person vulnerable to chronic lung infections. The disorder also causes the pancreas (an organ that produces enzymes that help digest food) to fail to produce digestive enzymes.

Most people with cystic fibrosis live into their 20s or 30s; some people live into their 40s or longer. Survival can be significantly improved with early diagnosis; consistent, effective treatment to manage the fluid buildup in the lungs; and good nutrition. Researchers are experimenting with ways to replace the defective genes with healthy ones.

Symptoms

Some infants who have cystic fibrosis have symptoms right after birth, while others may not have symptoms for several months or years. In infants, an early sign may be foul-smelling stool that is pale and greasy. Because the child’s body may not be absorbing sufficient nutrients from food, he or she may have inadequate weight gain or may lose weight. A child with cystic fibrosis may seem out of breath, have a constant cough that produces thick mucus, and have frequent lung problems, including pneumonia (see page 660), bronchitis (see page 655), and asthma (see page 640). He or she is more prone to dehydration, and his or her sweat may be excessively salty. Other complications include nasal polyps (see page 634) or rectal prolapse (see page 780). Damage to the pancreas can eventually lead to diabetes (see page 889).

Diagnosis

To diagnose cystic fibrosis, a doctor will order a sweat test, which can detect an abnormally large amount of chloride in the sweat, which is characteristic of cystic fibrosis. He or she may also order chest X-rays, a stool test, a mucus evaluation, and a blood test to detect the defective genes that produce cystic fibrosis. If prospective parents both carry the gene for cystic fibrosis, the disorder can be diagnosed during pregnancy with chorionic villus sampling, or CVS (see page 511), or amniocentesis (see page 510).

Treatment

To treat cystic fibrosis, a doctor prescribes pancreatic enzyme powders or pills to take with meals to aid digestion. Vitamin and nutritional supplements and a diet rich in calories and proteins help improve general health. A person may also need to take antibiotics for lung infections, decongestants to reduce congestion, and bronchodilator drugs to help open the airways. To help loosen and drain mucus from the lungs, the doctor will show you how to perform chest clapping (gentle pounding and pressing of the chest with a cupped hand) and postural drainage (see page 660).

Congenital Adrenal Hyperplasia

Congenital adrenal hyperplasia is a rare autosomal recessive genetic disorder that is present at birth. The disorder reduces the adrenal glands’ production of the hormone cortisol, which has a role in many body functions. An affected person is missing or deficient in an enzyme that is necessary for the production of cortisol. Many enzymes are involved in cortisol production, and if any of them are absent or deficient, cortisol production decreases, usually triggering an overproduction of male sex hormones (androgens). The absent or deficient enzyme can also cause the adrenal glands to reduce their production of the hormone aldosterone, which is necessary for balancing the amount