Humans differ not only at the level of DNA sequence. It has been recently discovered that humans can also differ in the number of copies of each gene. These are called copy number variants.

We carry two copies of most genes – one on the mother’s chromosome and one on the father’s chromosome. Occasionally alterations in a chromosome can lead to the gain or a loss of one copy. A deletion can occur when a fragment of DNA is lost – either during copying, or when the genes are shuffles during meiosis. By the same mechanisms, a duplication can occur, whereby which we gain an additional copy of a gene. Deletions and duplications of greater than 1,000 nucleotides are called copy number variants. A difference in the copy number of a gene can increase or decrease the level of that gene’s activity. For example, when a copy of a gene is deleted, the cell may produce half as much protein as compared to a normal cell. There are many diseases that are cause by changes in gene copy number. The most well-know is Down Syndrome, which is caused by having an extra copy of chromosome 21.

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