Humans are usually expected to have two copies of each segment of the autosomes - one inherited from each parent. However, it has been found that this may vary from chromosomal region to region and from person to person resulting in fewer or more copies of a particular segment than usual.
This is called copy number variation - the differences in the number of copies of a particular gene or segment of DNA due to gains or losses of around one thousand to several million base-pairs. These have been discovered by comparing genomes between people using DNA microarrays.
Copy number variations which include coding regions, and thus alter the number of copies of a gene present , are also sometimes referred to as changes in gene dosage.
Copy number variants may be inherited or may have occurred for the first time in the person whose genome is being examined. Some appear to cause no medical problems, but some CNVs have been associated with susceptibility or resistance to disease.
