All people have genes. They come in pairs. One copy is from the mother; the other copy is from the father. Genes are made of DNA which is a code. The code is not the same in every person. The differences are called variants. When a variant causes disease, it is often referred to as a mutation. Genetic contribution to disease is varied. When a mutation is present there can be different ways it can be passed down in a family:

• Autosomal dominant: This pattern of inheritance is caused by a genetic mutation 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 mutation and also developing the condition. Huntington’s disease is an example of this type of inheritance.
• Autosomal recessive: This pattern of inheritance is caused by mutations in both copies of the gene pair. People who have a mutation in just one copy of the gene are called carriers and do not have symptoms of the disease. If two people who are carriers have children together, each of their children would have a 25 percent chance of inheriting a mutation in each gene and developing the disease. Cystic fibrosis and sickle cell anemia are examples of this type of inheritance.
• X-linked: This pattern of inheritance is caused by a mutation on the X chromosome. Symptoms usually occur in males. An example is Duchenne muscular dystrophy.

Multifactorial Inheritance

Multifactorial inheritance refers to the pattern of inheritance of certain health problems caused by a combination of both genetic and environmental factors.

Certain risk genes have variants that do not cause disease but increase the risk for symptoms. Apolipoprotein (APOE) represents an example of a risk gene for Alzheimer’s disease.

Some disorders that “run in families” are due to multifactorial inheritance. 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.