Inherited, or genetic, diseases are caused by either chromosome abnormalities or specific alleles. The development of advanced techniques and new knowledge makes an understanding of genetic disease increasingly important.
Some genetic diseases are related to the presence of an additional chromosome or to the absence of a chromosome. These disorders result from errors that occur during meiotic cell division, causing some gametes to receive both members of a chromosome pair while other gametes receive neither member. If such gametes are involved in zygote formation, a genetic disorder occurs. The genetic damage usually is so severe that it causes a spontaneous abortion. In some cases, the effect is not lethal but disabling.
An example of a disabling genetic disorder is Down syndrome, one of the more common genetic disorders. It is caused by the presence of an extra chromosome 21. Down syndrome is characterized by mental retardation, short stature, short digits, slanted eyes, and a protruding tongue. Infants with Down syndrome are born more often to mothers and fathers over 40 years of age.
These disorders usually affect the infant’s metabolism after birth, when it must depend on its own life processes, or they may appear later in life. Consider a few examples of single-gene disorders.
Cystic fibrosis, an autosomal recessive disorder, is the most common genetic disorder among Caucasians. It is caused by a missing chloride channel on mucus-secreting cells. This causes production of thick mucus that blocks respiratory airways and leads to an early death from respiratory infections.
Phenylketonuria (PKU), an autosomal recessive disorder, is due to a missing enzyme needed to metabolize phenylalanine (an amino acid). Without treatment, mental and physical retardation result. A special diet that limits phenylalanine can prevent these effects if it is started at birth and continued to adulthood.
Tay-Sachs disease, an autosomal recessive disorder, primarily affects Jewish people of central European ancestry. An enzyme needed to metabolize a fatty substance associated with neurons is missing. The results are mental retardation, muscle weakness, seizures, and finally death, usually by two years of age.
Huntington disease, an autosomal dominant disorder, results from one or more missing enzymes needed in cellular respiration. This causes a buildup of lactic acid in neurons in the brain. Uncontrollable muscle contractions, memory loss, and personality changes begin between 30 and 50 years of age. Death occurs within 15 years after the appearance of symptoms.
Hemophilia A and hemophilia B, X-linked recessive disorders, result from missing clotting factors. Prolonged bleeding can be life threatening, and joints may be painfully disabled. Patients are dependent upon frequent transfusions of normal plasma or intravenous injections of the missing clotting factor.
Prospective parents who have genetic disorders in one or both of their families may benefit from genetic counseling. By collecting genetic information, a genetic counselor can inform prospective parents of the probability of a genetic disorder’s appearance in their children. Genetic information may be collected from family histories and blood tests of the prospective parents and family members. If the woman is pregnant, ultrasound may be used to detect gross fetal abnormalities, and fetal cells may be obtained for examination. Fetal cells are obtained in two ways: amniocentesis and chorionic villi sampling.
In amniocentesis, a hollow needle is inserted through the abdominal and uterine walls of the mother and into the amnion to draw out a sample of amniotic fluid. Fetal cells in the sample are grown by tissue culture and are analyzed to see if there are chromosomal abnormalities. Also, the amniotic fluid is analyzed for the presence of specific proteins that indicate serious neural defects. Amniocentesis is usually done around the 14th week of development, when ample amniotic fluid is available for sampling without injury to the fetus.
In chorionic villi sampling, a narrow tube is inserted through the cervix and fetal tissue from chorionic villi is suctioned out. Chromosomes in the fetal cells are examined for abnormalities. Chorionic villi sampling may be performed at 10 weeks and chromosome examination can be done immediately.
Both amniocentesis and chorionic villi sampling have inherent risks for mother and fetus. Fetal risks seem to be greater in chorionic villi sampling.
Fetal cells can also be collected for analysis through a procedure called fetal cell sorting. A small amount of fetal cells enter the mother’s blood supply during pregnancy. A fluorescent cell sorter can be used to identify and separate out the rare fetal cells from a maternal blood sample for analysis. This method of fetal cell collection circumvents the health risks associated with amniocentesis and chorionic villi sampling. Though these conventional methods are still more commonly used, fetal cell sorting is being used for purposes of sex determination and determination of fetal Rh status, in addition to the detection of major chromosomal abnormalities and some single-gene disorders.