The short answer
Mitochondria are the parts of a cell that make energy, and they carry a tiny amount of DNA of their own. That DNA comes almost entirely from the egg, so a mother can pass a mitochondrial change to sons and daughters alike, and a father cannot pass it on at all. If all of a mother's mitochondrial DNA carries the change, all her children inherit it. If only some of it does, different children can inherit very different amounts — which is why one child can be much more affected than another.
A different kind of DNA
Nearly all of your DNA sits in the middle of each cell. But mitochondria — the tiny power stations inside cells — carry a small separate set of their own.
An embryo gets its mitochondria from the egg. The few that arrive in the sperm are normally destroyed. That one fact produces the whole pattern:
| Parent with a mitochondrial DNA change | Children who can inherit it |
|---|---|
| Mother | Sons and daughters |
| Father | None |
This is not the same as X-linked inheritance, where a father does pass his X to every daughter. A father passes mitochondrial DNA to nobody.
What it looks like across three generations
Picture a grandmother who carries a mitochondrial DNA change. She has a daughter and a son, and both inherit some of it.
- The daughter’s children can inherit it from her.
- The son’s children cannot inherit it from him.
- The daughter’s children may be affected quite differently from one another.
Go one more generation and the same rule applies: a granddaughter can pass it on, a grandson cannot. That unbroken line through mothers — and the dead end at every man — is the thing to look for.
Why two children can be affected so differently
Each cell holds many mitochondria and many copies of mitochondrial DNA. Sometimes almost every copy carries the change. Sometimes it is a mixture of changed and unchanged copies. Doctors call that mixture heteroplasmy.
When eggs are made, each one gets its own sample of the mother’s mitochondrial DNA. Two eggs can end up with very different amounts of the changed copies — so one child inherits a lot and another very little.
The amount also varies from organ to organ. This is why a blood test does not always reflect what is happening in the muscle, brain or eye that is causing the symptoms.
Inheriting the change is not the same as becoming ill
Many mitochondrial conditions have a threshold. Cells cope with a certain amount of changed DNA, and symptoms appear only once energy production drops below what an organ needs. The brain, muscles, heart and eyes usually show it first because they use the most energy.
So there are really three separate questions:
- Did the child inherit the change?
- How much of it did they inherit?
- Is that amount high enough, in the organs that matter, to cause symptoms?
The family tree can help with the first. It usually cannot answer the other two.
Conditions you may have been told about
Named examples help, but “mitochondrial condition” and “inherited from the mother” are not the same thing. Some conditions that affect mitochondria are caused by ordinary genes in the middle of the cell and follow dominant, recessive or X-linked rules instead.
- LHON (Leber hereditary optic neuropathy) mainly affects the nerve carrying vision from the eye to the brain. It is caused by mitochondrial DNA changes, though not everyone who inherits one loses vision.
- MELAS can involve muscle weakness, seizures, headaches and stroke-like episodes. Mitochondrial DNA changes are among the common causes.
- MERRF often involves muscle jerks, seizures and weakness, and can vary enormously within one family.
The name of the condition alone is not enough to work out a family’s risk. What matters is the exact gene and change written on the laboratory report.
Who should be tested first
When a mitochondrial condition is suspected, it is usually more useful to test someone with clear symptoms than to start with a healthy relative. A blood sample is not always the right sample — sometimes muscle or urine gives a clearer answer.
Once the family’s exact change is known, everyone else can have a focused test instead of a broad search.
Common questions
- Can a father pass on a mitochondrial condition?
- Not when it is caused by a change in mitochondrial DNA — an embryo's mitochondria come almost entirely from the egg. Some conditions that damage mitochondria are caused by ordinary genes instead, and those can be passed on by either parent.
- Does an affected mother pass it to every child?
- If all of her mitochondrial DNA carries the change, yes. If she has a mixture of changed and unchanged copies, different children can inherit very different amounts.
- Why do brothers and sisters have such different symptoms?
- They may have inherited different amounts of the changed DNA. The amount also varies between organs, and the organs that use the most energy — brain, muscle, heart and eyes — usually show it first.
- Can sons be affected as badly as daughters?
- Yes. Both can inherit the change and become ill. The difference is only in what happens next: daughters can pass it to their own children, sons cannot.
- Are there tests during pregnancy?
- Sometimes, but they are more complicated than for an ordinary single gene, because the amount of changed DNA can differ between eggs. A specialist mitochondrial genetics service can explain what is possible and what the results could and could not tell you.