Sometimes a change occurs on a gene that causes it to stop working properly. This change is called a mutation. Such changes in the genetic sequence can

1. Inactivate the gene and result in a loss of function,
2. Cause the gene to create abnormal forms of the normal protein or
3. Result in increased production of the normal protein.

Genetic disorders can be caused by disease-causing mutations in a single gene or changes in a number of genes combined with environmental and lifestyle factors. Genetic mutations can be acquired during your lifetime (through exposure to radiation, certain chemicals, etc.) or can be inherited from one or both of your parents.

Diseases that are caused by a change in a single gene can be inherited in one of several ways, including:

• Autosomal dominant: This means the condition is caused by a genetic change in just one copy of the gene. Each child of a parent with an autosomal dominant condition has a 50 percent chance of inheriting the change and also developing the condition. Huntington's disease is an example of this type of inheritance.
• Autosomal recessive: In contrast to autosomal dominant inheritance, conditions that are autosomal recessive are due to a change in both copies of a gene. People who have a change in just one copy of the gene are called carriers and are not themselves affected. If two people who are carriers decide to have children together, each of their children would have a 25 percent chance of being affected with the condition. In order to be affected with the condition, a child would need to inherit the genetic change from both parents. Cystic fibrosis and sickle cell anemia are examples of this type of inheritance.
• Susceptibility gene: Some genes predispose to a condition but do not invariably lead to the disease. Apolipoprotein E4 represents an example of a susceptibility gene for Alzheimer's disease. Susceptibility genes for frontotemporal dementia are being sought.

Some genetic disorders that "run in families" are due to multifactorial inheritance, meaning they are associated with a combination of both genetic and environmental factors. Although complex disorders such as diabetes and heart disease often cluster in families, they do not have a clear-cut pattern of inheritance. This makes it difficult to determine a person’s risk of developing the condition in the future.