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). There are four different types of genetic disorders: (1) single-gene, (2) multifactorial, (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.
- Genetic Disorders: Types of Inheritance - From Gene Stories provided by BBC.
- Inheritance Patterns of Monogenic Disorders - From the Genetic Interest Group in the U.K.
- Genetics - From the Medical Encyclopedia at MEDLINEplus.
(2) Multifactorial (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 disorders. 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 rejoinings (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 nonchromosomal DNA of mitochondria. Mitochondria are small round or rod-like organelles that are 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.
http://www.ornl.gov/sci/techresources/Human_Genome/medicine/assist.shtml
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