Early and late onset somatic mutations.

<p>In the human cell lineage, germline progenitor cells that produce the gametes (at the horizontal dashed line) are formed before the somatic cell lineages start dividing. The progenitor cells and their daughter cells for 4 somatic cell lines are shown. (A) Here a somatic mutation has occurred in a CNS progenitor cell, which is passed on to its daughter cells (red-filled circles) but which leaves the other cell lines intact. Comparing DNA from both the CNS and from white blood cells will reveal this somatic mutation. (B) The somatic mutation here has affected an early stem cell, so most or all of the somatic cell lines will contain the mutation. This mutation can be detected if one monozygous twin has a white blood cell mutation and the other does not.