In general, people have two copies of each gene, one copy inherited from each parent. However, in some cases, the copies of genes vary from nil to one, three or more. These types of genetic differences are known as copy number variations (CNV). CNV can occur as a result of genetic variations (insertions, deletions, etc.) that happen at the DNA segments.
CNV count is different for every individual. More than 10% of the human genome appears to contain differences in gene copy number. While the variation does not necessarily affect a person’s health or development, some differences do influence a person’s risk of disease, particularly there is increased risk for certain types of cancer, or how they respond to certain drugs.