Scientists have proved that nobody is perfect by finding that an average healthy person carries around 400 potentially damaging DNA variants.
Two of these are likely to be defects directly linked to illness.
Researchers came up with the figure after matching whole genetic codes of 179 people against a database of disease-causing mutations.
In many cases the variants were “recessive” and unlikely to cause harm unless two copies are present, one in each pair of chromosomes.
But in one in ten people the effect was potentially more serious. These individuals either had two copies of a recessive variant, or a “dominant” mutation strong enough to act on its own.
The research is published online today in the American Journal of Human Genetics.
Study leader Professor David Cooper, from the University of Cardiff, said: “In the majority of people we found to have a potential disease-causing mutation, the genetic condition is actually quite mild, or would only become apparent in the later decades of life. We now know that normal, healthy people can possess many damaged or even completely inactivated proteins without any noticeable impact on their health.
“It is extremely difficult to predict the clinical consequences of a given genetic variant, but databases such as HGMD (human gene mutation database) promise to come into their own as we enter the new era of personalised medicine.”
DNA samples collected for the study were anonymised so participants could not learn the worrying news that they might be at risk from a genetic disorder.
The possibility of uncovering information that might cause people distress is a growing ethical issue for medical geneticists.