Many diseases are caused by genetic defects. The corresponding alleles may be dominant or recessive and may be found on autosomes or on sex chromosomes. Some genetic diseases are associated with extra chromosomes or missing pieces. An example of a dominant autosomal genetic disease is Huntington's disease. It results in neurological degradation leading to death. Because the symptoms do not appear until a person is about 40 years old, the victim may already have produced a family, potentially passing the trait on. The children of the victim are left knowing that they have a 50% chance of having the disease themselves. Since the 1980s, it has been possible to test these offspring for the gene, which, however, results in several types of ethical dilemmas (see Problem 6.2).
Many genetic diseases are caused by recessive genes on autosomal chromosomes. Albinos have a recessive defect on chromosome 11, which prevents formation of pigments in skin, hair, and eyes. Cystic fibrosis is caused by a chromosome 7 gene, which results in a defect in a membrane protein. Affected individuals produce excessive mucus, which harms respiratory and digestive function. One in every 11,000 persons born in this country inherits a defect in chromosome 12, resulting in phenylketonuria. Those affected lack an enzyme to metabolize the amino acid phenylalanine. The condition exhibits partial dominance. Heterozygous individuals have about 30% of the enzyme activity as normal people. Unchecked, it can cause retardation. However, routine screening at birth makes it possible to avoid disease symptoms by careful dietary control. In addition, affected people must avoid foods containing the artificial sweetener Aspartame, which is an amino acid dimer of phenylalanine and aspartic acid.
Several conditions are caused by recessive alleles on the X chromosomes. Since males have only one copy of X, they can inherit these conditions only from their mother. Examples of these conditions are color blindness for green, inability to form the enzyme glucose-6-phosphate dehydrogenase, Duchenne muscular dystrophy, and two forms of hemophilia. Since many X-linked genetic diseases are fatal when homozygous, only heterozygous females survive gestation. They themselves do not exhibit the disease and are called carriers. They pass the gene for the disease to (on average) half their daughters, who also become carriers, and to half their sons, who exhibit symptoms of the disease.
Another type of genetic disease is that related to chromosomal abnormalities such as extra or missing chromosomes, chromosome breakage, or translocation (the exchange of genetic material between chromosomes). Errors during meiosis can cause extra or missing chromosomes. A person with a single X chromosome and no Y becomes a sterile female. Some people have one or more extra sex chromosomes. One in 1000 males is XXY. These individuals are sterile, have tall stature and diminished verbal skills, yet normal intelligence. One in 1200 females is XXX. Many are normal, but some exhibit sterility and mental retardation. XYY males tend to be above average in height and below normal in intelligence. Prison inmates have been found to have a higher incidence of this disorder, although XYY males in the general population do not show any unusual tendencies.
Missing autosomes are usually fatal before birth, and extra autosomes are fatal in infancy, with the exception of the smallest autosome, number 21. Having three chromosomes is called trisomy. Trisomy 21 is the cause of 95% of Down syndrome. Affected persons show numerous characteristics, including an eyefold similar to those of oriental people, short stature, and small heads. They are retarded and susceptible to various diseases, including respiratory and heart problems, and to Alzheimer's disease in older persons. The rate of Down births increases rapidly with the age of the mother, from 1 in 1000 at age 30, to 1 in 100 at age 40, and 1 in 50 at age 45.
Cancer is a result of a series of mutations (see Section 6.2.3). If some of these are inherited, a person is more susceptible to cancer, since fewer mutations remain to occur. For example, a gene associated with breast cancer has been discovered on chromosome 17 that is responsible for about 10% of all cases. Women with the mutation have an 80 to 90% chance of contracting the disease.
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