Many types of genetic variations lend diversity to the gene pool, however, some genetic variations play a role in disease. Genetic variations that cause genetic disorders, also called mutations, can occur in a number of different ways and may affect varying amounts of genetic material.
- Single gene disorders (monogenic) occur as the result of genetic variations within a single disease-associated gene. The variation may be as small as a single base pair change or as large as the entire gene. Some single gene disorders are caused by having only one faulty copy of the gene (dominant) while others only occur when both copies are faulty (recessive). Some examples of monogenic disorders include sickle cell disease, cystic fibrosis and hemophilia.
- Complex or multifactorial disorders are those that are caused by interactions between multiple genes or by interactions between genes and environmental factors (diet, activity level, exposures, etc.). Although multifactorial disorders have a genetic basis, significant differences in disease severity (even within a family) may occur. Examples include heart disease, diabetes, and obesity.
- Chromosome disorders occur when there are extra, missing, or structurally altered chromosomes. Down syndrome is a chromosome disorder in which there is an extra (third) chromosome 21 (trisomy 21).