Genetic Conditions
Learning about mitochondrial inheritance
Genes are the instructions that tell your body how to develop and function. They are like a recipe for making cookies or the blueprints for a house. More than 99% of our genes are found in a part of our cells called the nucleus. These genes are called nuclear genes. A small number of genes are found in a different part of the cell called the mitochondria. These genes are called mitochondrial genes.
Variation
We all have differences in our genetic instructions that make us different from each other. We call these differences variants. Many of these differences do not cause any health problems. These are called benign variants. But sometimes differences in our genetic instructions can cause health problems. These differences are called pathogenic variants or disease-causing variants.
Sometimes a pathogenic variant is only seen in a portion of a person’s mitochondria. This is called heteroplasmy. The degree of heteroplasmy (how many mitochondria have a particular variant) can vary between body parts in a person. When enough mitochondria in a person’s body part have a pathogenic variant, then they may develop a mitochondrial condition. This is called the threshold effect.
Mitochondrial conditions
Mitochondrial conditions can affect people at different stages of their life. Depending on the condition, signs may be present prior to birth, at birth, in childhood, in adolescence, or in adulthood. The symptoms of mitochondrial conditions depend on the number of mitochondria within a cell that have the pathogenic variant, and which parts of the body are affected by the condition. This means that symptoms can sometimes be different from person to person.
There are hundreds of different mitochondrial conditions, each with different but overlapping symptoms. Mitochondrial conditions can be caused by pathogenic variants in mitochondrial genes or in nuclear genes. Genetic or metabolic specialists can help your healthcare provider understand if you have a mitochondrial condition.
Genetic testing for mitochondrial conditions
Genetic testing can help your healthcare team diagnose some mitochondrial conditions. In 2025, about half of all mitochondrial conditions can be diagnosed by genetic testing.
Depending on your specific health issues, family history, and the results of any previous lab and imaging studies, your healthcare provider may recommend a genetic test. Some types of mitochondrial variants are hard to detect in blood, so the genetic test may use a different sample, like urine (pee).
How mitochondrial genes are passed down from parent to child
A baby is formed by an egg and a sperm at conception. You get most genes (the nuclear genes) from both the egg and sperm that formed you. Mitochondrial genes, however, are only received from the egg, and almost never from the sperm. This means that you get mitochondrial genes only from your biological mother, which is called maternal inheritance. If the biological mother has a mitochondrial genetic variant, the variant will usually be passed down to all their children.
The risk to children depends on the specific variant, the degree of heteroplasmy in the biological mother, and other factors. And not all mitochondrial conditions are caused by pathogenic variants in mitochondrial genes. Some are caused by pathogenic variants in nuclear genes, which are inherited differently. So it can be difficult to predict whether and how a child will be affected by a mitochondrial condition.
Parents cannot control what genes are passed on to their children. Having a genetic condition, or having a child with a genetic condition, is not caused by anything a person or parent did or did not do. The risk of inheriting a mitochondrial condition is also not impacted by:
- the child’s sex
- anything a parent did or did not do before or during pregnancy
Finding a genetic variant for the first time: De novo variants
A person with a mitochondrial condition may or may not have a family history of the condition. In some families, the genetic variant responsible for the condition has been present in the family for generations. Other times, the genetic variant can be found in an individual for the first time. This is called a de novo (or “new”) variant. Once a de novo variant has occurred, the risk to their own child depends on whether the person with the de novo variant is male or female. This is because mitochondrial genes are only received from the egg cell, and almost never from the sperm.
Genetic counselling
Genetic counselling is available if you have a mitochondrial condition or if you are at risk of a mitochondrial condition.
A genetic counsellor can review your family history, genetic testing options, and screening options, and they can help families and individuals adjust to their diagnosis. Mitochondrial conditions may not always follow a mitochondrial inheritance pattern, so ask your healthcare provider if a referral for genetic counselling may be right for you.
If you have any questions about mitochondrial conditions or inheritance, talk with your healthcare team.
To see this information online and learn more, visit MyHealth.Alberta.ca/health/aftercareinformation/pages/conditions.aspx?hwid=custom.ab_genetics_mitochondrial_inhert_inst.
For 24/7 nurse advice and general health information call Health Link at 811.
Current as of: May 13, 2025
Author: Clinical and Metabolic Genetics, Alberta Health Services
This material is not a substitute for the advice of a qualified health professional. This material is intended for general information only and is provided on an "as is", "where is" basis. Although reasonable efforts were made to confirm the accuracy of the information, Alberta Health Services does not make any representation or warranty, express, implied or statutory, as to the accuracy, reliability, completeness, applicability or fitness for a particular purpose of such information. Alberta Health Services expressly disclaims all liability for the use of these materials, and for any claims, actions, demands or suits arising from such use.
