Autosomal Dominant, Multifactorial and Chromosomal Abnormalities
Another category of hereditary disease is called autosomal dominant disease. Because the defective gene is dominant, the disease is expressed even if only one gene is defective. A normal gene cannot mask the harmful effects of an abnormal gene as it can in autosomal recessive disease. If one parent has an autosomal dominant disease, the chances are 50 percent that each child will inherit the disorder.
Huntington chorea: An example of an autosomal dominant disease is Huntington chorea, a brain disease marked by abnormal body movements and mental deterioration beginning in middle age.
Although a few medicines have been found to make the symptoms more tolerable, the disease has no cure. Researchers have been able to identify carriers in families with Huntington chorea with a genetic test. It is hoped this procedure will be perfected and also extended to prenatal diagnosis.
Multifactorial Genetic Diseases
Multifactorial genetic diseases are illnesses that tend to run in families. These diseases are not due simply to the inheritance of a single defective gene. Rather, a cluster of faulty genes is inherited, which predisposes the person to a disease. Given the appropriate environmental factors, the person may actually develop that disease. Examples of illnesses that run in families include such chronic adult diseases as coronary heart disease, high blood pressure, and stomach ulcers, as well as birth defects, such as cleft lip and palate and spina bifida.
Cleft lip and palate: In cleft lip, the upper lip is divided by a vertical fissure. In cleft palate, the roof of the mouth is split by a longitudinal fissure. These two birth defects can occur alone or together. They are the result of incomplete fusion of the components that form the lip and mouth during fetal development.
Spina bifida: Spina bifida is a failure in the closure of the bony vertebral column with or without protrusion of the nerve tissue of the spinal cord. Paralysis below the defect often accompanies spina bifida if the spinal cord does protrude. When the spinal cord does not protrude, the vertebral defect may go unnoticed. Clues to indicate the presence of this form of spina bifida are abnormalities of the skin and tufts of hair overlying the spine in the lower part of the back.
During pregnancy, spina bifida in the fetus can be diagnosed by means of ultrasound study and detection of elevated levels of a substance called alpha-fetoprotein in the mother's blood and in the amniotic fluid that bathes the fetus. Spina bifida can be caused by a folate deficiency in the mother during pregnancy or exposure of the pregnant woman to certain drugs that interfere with folate.
Sometimes the structure or the number of chromosomes is not normal. The risk of having a child with chromosomal abnormalities increases with increasing maternal age (and to some extent with increasing paternal age as well). If chromosomal abnormalities occur in the sex cells (eggs and sperm), the offspring may have physical and mental disorders.
Down syndrome: Formerly called mongolism, Down syndrome is a condition caused by a chromosomal abnormality. Due to the failure of the chromosomes to divide evenly during cell division, the person with Down syndrome has an extra chromosome (a total of 47). The presence of this extra chromosome causes a characteristic physical appearance and delayed physical and mental development. Other ailments, such as defects in the heart and digestive system, can accompany this syndrome. Despite their disabilities, children with Down syndrome usually have pleasant dispositions and can do quite well if given special therapy.
The cause of the chromosomal abnormality leading to Down syndrome is unknown. A genetic predisposition may exist. The incidence of Down syndrome increases with increasing maternal age. The condition can be diagnosed prenatally with the use of amniocentesis. For these reasons, women who have previously given birth to a baby with a chromosomal abnormality or who are older than 35 years of age are encouraged to undergo amniocentesis.