For a child to have a genetic mutation disease, both parents must pass on a defective gene for that disease. This means that the child carries two defective genes for the disease. If a person with the disease has children, they can only pass on a defective gene for the specific disease because both of their genes for that disease are defective.

The term “carrier” usually refers to individuals who have only one defective gene and do not have the disease themselves. However, if someone has the genetic disease, they are carrying the genes for it.

In some cases, diseases caused by mutations in genes are not genetic in the sense of inheriting a defective gene from a parent. Mutations can occur due to factors such as smoking, drug use, heavy alcohol consumption, obesity, uncontrolled diabetes, certain medications, nutritional deficiencies, environmental pollution, or unknown causes. These mutations are not hereditary and are typically referred to as syndromes, conditions, or birth defects instead of diseases.

For instance, most cases of Down syndrome are not inherited and do not run in families. Only about 1% of cases are caused by an inherited genetic mutation.

It is possible that your friend does not have an inherited gene mutation disease but rather an acquired genetic mutation condition, which may explain why it is unknown if they are a carrier.