Why should mutations increase as fathers’ age? The answer lies in the life history of the sperm. By the time a man is 40, each of his sperm cell precursors, called spermatogonia, has divided approximately 660 times, or about 23 times a year after puberty, in order to give rise to sperm [The older a man is, the more spermatogonia divisions occur]. By contrast, in a female, egg precursor cells divide only 24 times, all but one of these divisions occurring before she is born. The more replication, the greater the chance that a copying error – a mutation – will occur. To compound matters, DNA-repair enzymes become less efficient as a man ages and more frequently fail to fix a mutant sperm.