Genetics Northern California

Autosomal Dominant

“Autosomal” means that there are two copies of a particular gene, regardless of a person's sex.  One copy is from the egg and one copy is from the sperm.    

“Dominant” means that a change in just one copy of a gene can lead to a genetic disorder or a genetic risk for medical problems. Some genes in our body need to have both copies working correctly. When one copy is not working or is working poorly, it can lead to medical issues. 

How is an autosomal dominant (AD) condition inherited? 

Click here for a handout on autosomal dominant inheritance. 

An AD condition may be inherited from a parent when they have a harmful change (variant) in an AD gene. Either parent can pass an AD condition to a child. In every pregnancy, there is a 1 in 2 (50%) chance of passing on the variant.   

Sometimes an AD condition is due to a brand new mutation in a gene. The genetic term is a “de novo” mutation. A de novo mutation often happens as a random DNA change when an egg or sperm is being formed, or in an early embryo. A person with a de novo mutation is the first person in the family to have the condition. Even though the condition was not inherited from a parent, there is a 50% chance to pass it on. 

Genetic terms related to AD conditions:

  • Variable expressivity: The same variant in a gene can sometimes cause many different medical issues. One person may have mild symptoms, while another with the same variant may have more severe medical problems. This means family members with the same AD condition could show a wide range of symptoms.   
  • Reduced penetrance:  A person who inherits a variant in an AD gene may not develop any signs or symptoms of the condition. The chance of developing the condition depends on the specific gene and variant.  
  • Gonadal mosaicism: This is rare. Most people with an AD condition carry the disease-causing variant in every cell of their body. However, sometimes the variant is only present in the egg or sperm cells and not in other cells of the body.  This is called gonadal mosaicism. A person with gonadal mosaicism most does not have any features of the condition. But the variant can be passed from the egg or sperm cells to that person's offspring.  

Last reviewed: March 2022
Reviewed by: Kimberly Barr, MS, LCGC; Kaiser Permanente Genetics Department