Sometimes small changes or mistakes are introduced into a DNA sequence. These changes are called mutations . They account for many of the differences in our genomes, and for much of the diversity among our species. Sometimes mutations occur when DNA is copied from parent to child. They also happen by accident as the body ages or when living cells are exposed to radiation or toxins.
Some mutations happen in genes.
A mutation in a gene might have no effect on a person, or it might cause a gene to be unreadable or to say something with a different meaning. Some of these changes in meaning can cause genetic diseases. That is, the gene and the marker are usually inherited together when passed from parent tochild. Scientists can use these linked markers to identify the presence of a gene even if the gene itself has not been found. The human genome is about three-billion 3,000,000,000 bases long.