Genetic diseases can be examined under five main headings:
- Single-Gene Diseases: Result from a mutation in a single gene. Diseases such as cystic fibrosis, thalassemia and hemophilia develop when a specific gene is disrupted and are usually inherited within the family.
- Chromosomal Diseases: Result from changes in the number or structure of chromosomes. Down syndrome, for example, is associated with an extra copy of chromosome 21. These diseases usually arise from incorrect division or joining of chromosomes and can cause serious health problems.
- Multigenic/Multifactorial Diseases: Result from the combined effect of multiple genes and environmental factors. In the development of diseases such as cancer, diabetes and autism, lifestyle and environmental influences play as much of a role as genetic factors.
- Mitochondrial Diseases: Result from genetic defects in the mitochondria, which are responsible for the cells' energy production. Mutations in mitochondrial DNA disrupt energy production and can cause problems such as muscle weakness, neurological problems and heart disease. They are usually inherited from the mother.
- Epigenetic Diseases: Result from changes in the way genes are expressed without any change in the DNA sequence. Epigenetic disorders can lead to cancer, obesity, heart disease and neurological disease; environmental factors and lifestyle can trigger epigenetic changes.