What causes a genetic disease?

Genes are made up of deoxyribonucleic acid (DNA) and come in pairs, one from each parent. Genes store the code—the information and instructions—the body needs to make proteins. Nearly every function of the body is made possible by proteins. When a piece of DNA is missing or changed, it can alter protein production. The protein may no longer be able to carry out its normal function.

Even if a single protein is missing, in short supply, or made wrong, the effect on the body can be harmful. Some genetic diseases are caused by a malfunctioning or missing gene or genes.

A genetic disease can be passed down from one or both parents or can be a result of random errors in the body’s genes.

Complex diseases

Complex diseases are caused by a combination of multiple genetic abnormalities and environmental and lifestyle factors. While it’s common for many family members to suffer from the same complex disease, complex diseases aren’t directly inherited. Instead, genes may be inherited that increase a person’s risk of developing a complex disease, but whether that person develops the disease or not is influenced by other genetic factors in addition to environmental factors, like exposure to pollutants, carcinogens, or infectious agents, and lifestyle factors, such as diet, lack of exercise, and smoking. 

Most diseases that people suffer from are considered complex diseases, like cancer, cardiovascular diseases, and autoimmune diseases, and they represent an important area of focus for treatment advancement.  

Researchers are exploring gene therapies that may help improve outcomes for people suffering from complex diseases. Some treatments have already been approved to treat cancer.

Monogenic diseases

Monogenic diseases are caused by a single gene that is missing or malfunctioning. The mutated gene is inherited, meaning it is passed from parent to child. Many times, a person may have the mutation and not have any symptoms. These people are known as carriers and can unknowingly pass down the faulty gene to their children. 

There are as many as 8000 monogenic diseases, so while each individual monogenic disease is rare, it’s estimated that about 6% of people will at some point be affected by a monogenic disease. 

Monogenic diseases often lead to premature death, and historically, treatment has focused on managing symptoms rather than targeting the genetic root cause.