Single-Gene Disorders
Single-gene disorders, also called monogenic disorders, are defects caused by a mutant allele of a single gene that results in the functional loss of a protein. Single-gene disorders are generally easy to trace because they follow the rules of classical Medelian genetics. Single-gene disorders can be autosomal or X-linked, dominant or recessive and usually become evident by studying a particular family's pedigree. Common single-gene disorders include sickle cell anemia, familial hypercholesterolemia, Duchenne muscular dystrophy, and cystic fibrosis. Some speech and language disorders are also caused by the interference of a single gene.
Mutations in DNA (deoxyribonucleic acid) arise because of a spontaneous chemical change that results in a substitution, deletion, or insertion of a nucleotide base pair. As a result, many mutations cause physical or mental disorders. Sickle cell anemia is caused by a nucleotide base substitution that results in a defective hemoglobin molecule. Because sickle cell anemia is an autosomal recessive disorder, individuals who have the disease must have two abnormal copies of the gene, one from each parent. Familial hypercholesterolemia (FHC) is due to a defective gene on chromosome 19 that codes for an abnormal receptor protein unable to mediate the uptake of cholesterol into cells. As a result, there is a cholesterol buildup in the blood that may eventually lead to arteriosclerosis or coronary heart disease. FHC is an autosomal dominant disorder where only one abnormal allele is necessary to have the disease. Duchenne muscular dystrophy is due to an X-linked recessive, abnormal gene that is much more frequent among males than females. Males only need one mutant allele to have the disease because they only have one X chromosome; females do not express the disease unless both X chromosomes are affected. The disease is caused by a lack of dystrophin, a protein found in the membranes of muscle fibers. Hypophosphatemia is an X-linked dominant disorder characterized by low levels of phosphorus in the blood. Because of a defective protein, the kidneys are unable to reabsorb phosphorus or calcium resulting in abnormal, fragile bones. Hypophosphatemia is a relatively rare disorder that affects both males and females.
In most single-gene disorders, adding a functional protein to affected cells would cure the disorder. Currently, doctors attempt to do this through gene therapy. Theoretically, if doctors can locate the mutant gene, they can replace it with a normal gene that will produce functional proteins, curing single-gene disorders such as cystic fibrosis (CF). Normal CF genes are manufactured in the laboratory and delivered into defective cells by microscopic fat capsules called liposomes. Scientists also use viruses as vehicles for transporting healthy genes into affected cells. However, most gene therapies are still in the experimental stage as scientists continue to work towards a cure for single-gene disorders.
The underlying genetics of speech and language are believed to be due to several genes. However, one such speech and language disorder has been linked to the disruption of a single gene. This gene abnormality exists on chromosome seven, where a gene linked to autism also exists. Autism is a polygenic disorder that affects the brain's ability to communicate with others and react well with the environment. Polygenic disorders involve the combination of alleles in more than one gene. Although these disorders are also inherited, they are far more complex than single-gene disorders.
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