Medical Health Encyclopedia

Genetics

In AUTOSOMAL RECESSIVE INHERITANCE, the parents of an affected individual may not express the disease. On average, the chance of an affected child's brothers or sisters having the disease are 1 in 4. Males and females are equally likely to be affected. For a child to have symptoms of an autosomal recessive disorder, the child must receive the defective gene from BOTH parents.

Because most recessive disorders are rare, a child is at increased risk of a recessive disease if the parents are related. Related individuals are more likely to have inherited the same rare gene from a common ancestor.

In X-LINKED RECESSIVE INHERITANCE, the incidence of the disease is much higher in males than females. Since the abnormal gene is carried on the X chromosome, males do not transmit it to their sons -- they do transmit it to their daughters.

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The presence of one normal X chromosome masks the effects of the X chromosome with the abnormal gene. So, almost all of the daughters of an affected man appear normal, but they are all carriers of the abnormal gene. The sons of these daughters then have a 50% chance of receiving the defective gene.

In X-LINKED DOMINANT INHERITANCE, the presence of the defective gene appears in females even if there is also a normal X chromosome present. Since males pass the Y chromosome to their sons, affected males will not have affected sons, but all of their daughters will be affected. Sons or daughters of affected females will have a 50% chance of getting the disease.

EXAMPLES OF SINGLE GENE DISORDERS

Autosomal recessive:

• Cystic fibrosis (CF) is a very common hereditary disorder (1 out of 2,000 Caucasian births). The normal function of a particular protein is to transport chloride ions into certain cells. Deficiency of this protein somehow results in the accumulation of thick mucus in the lungs and other parts of the body. This situation compromises respiration and greatly increases the chance of pulmonary infections. Affected individuals rarely survive to the age of 40.
• Phenylketonuria (PKU ) is a common genetic disorder (1 out of 12,000 births) which results from a deficient enzyme required for the metabolism of the amino acid phenylalanine. Failure to recognize the disorder early in life results in mental retardation. Many states require all newborns to be screened for this disease.
• Alpha-1-antitrypsin (AAT ) deficiency is a disorder seen in about 1 out of 10,000 births. The normal function of the protein is to inhibit enzymes which escape from white blood cells in the process of destroying invading bacteria. Affected individuals are much more likely to develop emphysema than usual.
• Sickle cell anemia is a disorder common in individuals with an African ethnic background. The high frequency of the gene probably relates to the fact that the heterozygotes are resistant to malaria. The homozygotes have a predominance of an abnormal hemoglobin in their red blood cells. This abnormal protein causes the red blood cells to assume abnormal shapes and to lyse (a process of disintegration or dissolution) in small blood vessels under conditions of reduced oxygen pressure.
• ADA deficiency is a rare immunodeficiency disorder, sometimes called the "boy in a bubble" disease, which results from the deficiency of an enzyme called adenosine deaminase. This enzyme is important for the normal function of lymphocytes which are the primary components of the immune system. This disease has the distinction of being the first to be treated effectively by genetic engineering. Some of the patient's cells are removed from the body, injected with a normal gene, then reintroduced to the body.