When we compare the DNA of two people, we find many small variations. These differences, called genetic variants, explain part of our diversity. Most cause no health problems. But some are known to be linked to hereditary diseases.
To find out what an ordinary human genome contains, researchers from the University of Exeter in the United Kingdom examined data from nearly 900,000 people. They used two large genetic databases, one British and the other American, without selecting only patients with diseases.
DNA
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The result is striking: compared with reference DNA used by scientists, each person generally has between 4.3 and 5.3 million small differences. The vast majority are common and probably have no noticeable effect on health.
The researchers also looked at rare variants that medical databases classify as responsible, or probably responsible, for certain genetic diseases. In the genes studied, each person carries an average of one to three.
Why are these people not necessarily sick? In many cases, we have two copies of a gene, one inherited from each parent. For certain diseases known as recessive, both copies must be affected for the disease to appear. A single copy carrying a variant can therefore have no consequences for the person.
This is how a healthy adult can carry a genetic variation associated with a disease and potentially pass it on to their children. Other variants cause disease only under certain circumstances.
This research mainly provides guidelines for better interpreting genetic tests. Detecting a variation associated with a disease is not, by itself, enough to conclude that the disease will develop.