Healthy mothers can harbour hidden cell mutations that may trigger potentially lethal mitochondrial diseases in their children, research has shown.
The new study reveals how devastating inherited disorders can appear suddenly in previously unaffected families.
Mitochondria are the tiny rod-shaped power plants in cells that generate energy. They contain their own private DNA, which is passed onto children by mothers but not fathers.
Inherited mutations in mitochondrial DNA are rare, affecting around one in 10,000 newborn babies, but can cause disabling and life-threatening conditions.
One example is Leigh Syndrome, a severe brain disorder which emerges in the first year of life and typically results in death within two to three years.
Lead scientist Professor Patrick Chinnery, the Medical Research Council's Mitochondrial Biology Unit at Cambridge University, said: "We know that these devastating mitochondrial mutations can pop up in families without any previous history, but previously we didn't know how that happened.
"We were surprised to find that egg cells in healthy females all carry a few defects in their mitochondrial DNA."
The researchers carried out tests on mitochondrial DNA in the cells from mouse and human female embryos that go on to develop into eggs.
They found a variety of mutations in the mitochondrial DNA in developing egg cells in all 12 of the human samples studied.
The study, published in the journal Nature Cell Biology, shows that low levels of mitochondrial mutations are carried by healthy humans.
Over time and successive generations the mutations can accumulate until they are severe enough to trigger disease, the scientists believe.
In each mature cell, a few faulty mitochondria could hide unnoticed among thousands of healthy neighbours.
Usually, the defective cells are eliminated by natural processes that are not completely understood.
Prof Chinnery added: "Unfortunately, the purification process is not perfect, and occasionally defective mitochondria leak through. This can cause a severe disease in a child, despite no one else in the family having been affected."
Mitochondrial mutations have been associated with brain diseases linked to ageing such as Alzheimer's and Parkinson's.
Previously it was assumed that such mutations arise over a person's life time. Now scientists believe some of these DNA faults may be inherited, thereby programming people to be at risk of end-of-life diseases when they are born.