National Institutes of Health U.S. Department of Health and Human Services
Frequently Asked Questions About Genetic Disorders
What are genetic disorders?
A genetic disorder is a disease caused in whole or in part by a change in the DNA sequence away from the normal sequence. Genetic disorders can be caused by a mutation in one gene (monogenic disorder), by mutations in multiple genes (multifactorial inheritance disorder), by a combination of gene mutations and environmental factors, or by damage to chromosomes (changes in the number or structure of entire chromosomes, the structures that carry genes).
As we unlock the secrets of the human genome (the complete set of human genes), we are learning that nearly all diseases have a genetic component. Some diseases are caused by mutations that are inherited from the parents and are present in an individual at birth, like sickle cell disease. Other diseases are caused by acquired mutations in a gene or group of genes that occur during a person's life. Such mutations are not inherited from a parent, but occur either randomly or due to some environmental exposure (such as cigarette smoke). These include many cancers, as well as some forms of neurofibromatosis.
Genetic disorders typically involve the inheritance of a particular mutated disease-causing gene, such as sickle cell disease, cystic fibrosis, and Tay-Sachs disease. The mutated gene is passed down through a family, and each generation of children can inherit the gene that causes the disease. Rarely, one of these monogenic diseases can occur spontaneously in a child when his/her parents do not have the disease gene, or there is no history of the disease in the family. This can result from a new mutation occurring in the egg or sperm that gave rise to that child.
Most genetic disorders, however, are "multifactorial inheritance disorders," meaning they are caused by a combination of inherited mutations in multiple genes, often acting together with environmental factors. Examples of such diseases include many commonly-occurring diseases, such as heart disease and diabetes, which are present in many people in different populations around the world.
Research on the human genome has shown that although many commonly occurring diseases are usually caused by inheritance of mutations in multiple genes at once, such common diseases can also be caused by rare hereditary mutations in a single gene. In these cases, gene mutations that cause or strongly predispose a person to these diseases run in a family, and can significantly increase each family member's risk of developing the disease. One example is breast cancer, where inheritance of a mutated BRCA1 or BRCA2 gene confers significant risk of developing the disease.
Geneticists group genetic disorders into three categories:
Monogenetic 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). Examples of monogenic disorders are: sickle cell disease, cystic fibrosis, polycystic kidney disease, and Tay-Sachs disease. Monogenic disorders are relatively rare in comparison with more commonly-occurring diseases, such as diabetes and heart disease. A major distinction among monogenic disorders is between "dominant" and "recessive" diseases. Dominant diseases are caused by the presence of the disease gene on just one of the two inherited parental chromosomes. In dominant diseases, the chance of a child inheriting the disease is 50 percent. In a family situation, for example, if the parents have four children, it may be possible that two of those children inherit the disease gene. Examples of dominant diseases are Huntington's disease and Marfan syndrome. Recessive diseases require the presence of the disease gene on both of the inherited parental chromosomes. In this case, the chance of a child inheriting a recessive disease is 25 percent. In the family example, if the parents have four children, it may be more likely that only one child will develop the disease. Examples of recessive diseases include cystic fibrosis and Tay-Sachs disease.
Multifactorial inheritance disorders are caused by a combination of small inherited variations in genes, often acting together with environmental factors. Heart disease, diabetes, and most cancers are examples of such disorders. Behaviors are also multifactorial, 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's disease.
Chromosome disorders are caused by an excess or deficiency of the genes that are located on chromosomes, or by structural changes within chromosomes. (See the NHGRI fact sheet about chromosome abnormalities; Down syndrome, for example, is caused by an extra copy of chromosome 21 (called trisomy 21), although no individual gene on the chromosome is abnormal. Prader-Willi syndrome, on the other hand, is caused by the absence or non-expression of a group of genes on chromosome 15. A specific form of blood cancer (chronic myeloid leukemia, CML) may be caused by a chromosomal translocation, in which portions of two chromosomes (chromosomes 9 and 22) are exchanged. No chromosomal material is gained or lost, but a new, abnormal gene is formed that leads to formation of cancer.
Genetic and Rare Diseases Information Center (GARD) 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.
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.)