All about Disease Causes and Symptoms
A genetic disorder is a disease caused by a different form of a gene called a variation, or an alteration of a gene called a mutation. Many diseases have a genetic aspect. Some, including many cancers, are caused by a mutation in a gene or group of genes in a person's cells. These mutations can occur randomly or because of an environmental exposure such as cigarette smoke.
Other genetic disorders are inherited. A mutated gene is passed down through a family and each generation of children can inherit the gene that causes the disease. Still other genetic disorders are due to problems with the number of packages of genes called chromosomes. In Down syndrome, for example, there is an extra copy of chromosome 21.
If you know that you have a genetic problem in your family, you can have genetic testing to see if your baby could be affected.
What are Genetic Disorders?
Both environmental and genetic factors have roles in the development of any disease. A genetic disorder is a disease caused by abnormalities in an individual’s genetic material (genome). The four different types of genetic disorders are(1) single-gene, (2) multi factorial, (3) chromosomal, and (4) mitochondrial.
(1) Single-gene (also called Mendelian or monogenic) - This type is caused by changes or mutations that occur in the DNA sequence of one gene. Genes code for proteins, the molecules that carry out most of the work, perform most life functions, and even make up the majority of cellular structures. When a gene is mutated so that its protein product can no longer carry out its normal function, a disorder can result. There are more than 6,000 known single-gene disorders, which occur in about 1 out of every 200 births. Some examples are cystic fibrosis, sickle cell anemia, Marfan syndrome, Huntington’s disease, and hereditary hemochromatosis.
Single-gene disorders are inherited in recognizable patterns: autosomal dominant, autosomal recessive, and X-linked. More information on the different modes of inheritance is available from the following Web sites:
- Inheritance of Single Gene Defects - From the The Merck Manual of Diagnosis and Therapy.
- Inheritance Patterns of Monogenic Disorders - From the Genetic Interest Group in the U.K.
- Genetics - From the Medical Encyclopedia at MEDLINEplus.
(2) Multi factorial (also called complex or polygenic) - This type is caused by a combination of environmental factors and mutations in multiple genes. For example, different genes that influence breast cancer susceptibility have been found on chromosomes 6, 11, 13, 14, 15, 17, and 22. Its more complicated nature makes it much more difficult to analyze than single-gene or chromosomal disorders. Some of the most common chronic disorders are multifactorial. Examples include heart disease, high blood pressure, Alzheimer’s disease, arthritis, diabetes, cancer, and obesity. Multifactorial inheritance also is associated with heritable traits such as fingerprint patterns, height, eye color, and skin color.
(3) Chromosomal - Chromosomes, distinct structures made up of DNA and protein, are located in the nucleus of each cell. Because chromosomes are carriers of genetic material, such abnormalities in chromosome structure as missing or extra copies or gross breaks and re joinings (translocations) can result in disease. Some types of major chromosomal abnormalities can be detected by microscopic examination. Down syndrome or trisomy 21 is a common disorder that occurs when a person has three copies of chromosome 21.
(4) Mitochondrial - This relatively rare type of genetic disorder is caused by mutations in the non chromosomal DNA of mitochondria. Mitochondria are small round or rod-like organelles involved in cellular respiration and found in the cytoplasm of plant and animal cells. Each mitochondrion may contain 5 to 10 circular pieces of DNA.
Why we have Genes that Cause Genetic Disorders?
Many genes are named for the disorders to which they have been linked. This can be very confusing. For example, the gene associated with hereditary hemochromatosis is called the “hemochromatosis gene.” This name implies that the gene exists for the sole purpose of causing disease, which of course is not the case. The normal function of a gene is to encode a protein, not cause illness. Disease occurs when genes are unable to work properly. The hemochromatosis gene actually codes for a membrane protein that works with other proteins to regulate iron absorption in cells. Like other single-gene disorders, hemochromatosis occurs when a gene is mutated in a way that prevents it from encoding a normal, functional protein product. See hereditary hemochromatosis disorder and gene profiles for more information about this condition.
Ethical, Legal, and Social Implications of Having a Genetic Disease
Are patients being properly informed about the risks and limitations of genetic technology? How does personal genetic information affect an individual and society's perceptions of that individual? Who owns and controls genetic information? If you are tested for or diagnosed with a genetic disorder and this information becomes a part of your medical record, insurance companies, employers, and other agencies may be able to access this information. Without adequate legal measures to protect individuals from misuse of their medical information, individuals diagnosed with certain genetic conditions potentially could be denied insurance coverage, employment, or other benefits.