http://www.ornl.gov/sci/techresources/Human_Genome/home.shtml
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.
PACER Center
Parent Advocacy Coalition for Educational Rights
The following links are from the PACER Center Web site at
http://www.pacer.org/links/national/disability.htm