The other kind of mutation that can occur in Y DNA is single point polymorphisms, or SNP�s. The term means that a single letter in the genetic code has changed, for instance from A to T, resulting in two possible values for the DNA at that location. In Y DNA, these mutations occur infrequently; maybe one every one or two thousand years, more or less.
Once a single point mutation occurs, it is handed down to all the descendants of the first man who carried that mutation, forever. This makes it possible to recognize all of the descendants of this man. This diagram helps show how SNP�s work. The little colored dots represent the SNP�s that each man carries. Each man who is descended from someone who carried a mutation, carries that mutation. In time some descendants come to have more mutations.
Over time, some people in each line of descent from a mutation, develop more mutations. Between the first people who have descendants living today, 60,000 years ago, and their descendants, there may be a large number of such mutations. Scientists track where and when these mutations happen. This enables them to trace the paths people followed as they spread around the world. A man who knows what SNP mutations he has can trace where his ancestors have lived for thousands of years and what they were doing, and knows a lot about how his particular ancestors got from one place to another.
SNP�s are called by names like M168 or L33.
