goes on to divide into two cells, then four, then eight cells of the growing embryo, the new mutations it has acquired will be faithfully copied into the genome of every descendant cell in that growing person’s body. Even the sex cells that create the embryo—the mother’s egg and the father’s sperm—have incorporated new mutations that never before existed in either family’s germline. As a result, each one of us begins life with fifty to a hundred random mutations that arose de novo (“anew”) in our parents’ germ cells.
