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Frequently Asked Questions About Genetic Disorders
What are genetic disorders?
A genetic disorder is a disease caused in whole or in part by a "variation" (a different form) or "mutation" (alteration) of a gene.
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.
Additional Resources
- Genetic and Rare Diseases Information Center
The Genetic and Rare Diseases Information Center (GARD) provides accurate, reliable and current information on genetic and rare diseases to patients and families, health care professionals and biomedical researchers. It was established by NHGRI and the Office of Rare Diseases (ORD).
- Genetic Alliance [geneticalliance.org]
The Genetic Alliance is an international coalition comprised of more than 600 advocacy, research, and healthcare organizations that represent millions of individuals with genetic conditions and their interests.
- What is a Genetic Disease? [geneticalliance.org]
More information from the Genetic Alliance
- Behavioral Genetics [ornl.gov]
Provides an overview of behavioral genetics
- Genetics Home Reference [ghr.nlm.nih.gov]
Provides consumer information about genetic conditions and the genes or chromosomes responsible for those conditions.
- Online Genetic Support Group Directory [mostgene.org]
Alphabetic index of genetic disorders with links to associations, support groups and disease-related resources. Hosted by Mountain States Genetic Network (MoSt.)
Last Reviewed: December 19, 2008
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