A genetic disorder is an illness caused by abnormalities in genes or chromosomes. While some diseases, such as cancer, are due in part to a genetic disorders, they can also be caused by environmental factors. Most disorders are quite rare and affect one person in every several thousands or millions. Some types of recessive gene disorders confer an advantage in the heterozygous state in certain environments.
As we unlock the secrets of the human genome, we are learning that nearly all diseases have a genetic component. Some, including many cancers, are caused by a mutation in a gene or group of genes in the cells of an individual. Such mutations can occur randomly or due to some environmental exposure (such as cigarette smoke).
Other genetic disorders are hereditary - such as Huntington disease or Marfan's disease - where a mutated gene is passed down through a family and each generation of children can inherit the gene that causes the disease.
But most genetic disorders are "multifactorial inheritance disorders," meaning they are caused by a combination of small variations in genes, often in concert with environmental factors.
Through research on the human genome, we now know that many common diseases usually caused by genetic alterations in the genes of an individual's cells - such as breast cancer and colon cancer - also have rare hereditary forms. In these cases, gene variants that cause or strongly predispose a person to these cancers run in a family and significantly increase each member's risk of developing the disease.
Geneticists group genetic disorders into three categories:
Single gene disorders are caused by a mutation in a single gene. The mutation may be present on one or both chromosomes (one chromosome inherited from each parent). Sickle cell disease, cystic fibrosis and Tay-Sachs disease are examples of single gene disorders.
Chromosome disorders are caused by an excess or deficiency of the genes that are located on chromosomes, or structural changes within chromosomes. Down syndrome, for example, is caused by an extra copy of chromosome 21, but no individual gene on the chromosome is abnormal.
Multifactorial inheritance disorders are caused by a combination of small variations in genes, often in concert with environmental factors. Heart disease and most cancers are examples of these disorders. Behaviors are multifactorial, complex traits involving multiple genes that are affected by a variety of other factors. Researchers are learning more about the genetic contribution to behavioral disorders such as alcoholism, obesity, mental illness and Alzheimer disease. There is currently no recommended genetic testing for behavioral disorders such as alcoholism or obesity.
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