"But in men, sperm are being made continuously throughout adult life from precursor germ cells that have much more time to be damaged..."

This is not well-known but this site reveals that testes have a preference for mutated sperm.


Sperm mutate more often as men get older - not because it helps their resulting offspring, but because it appears to help sperm survive in the testes, researchers have found. The discovery was made by accident - while seeking to understand a rare inherited condition known as Apert syndrome, which is more likely to develop in children born to older men, said the report in the journal Science."Striking male biases in mutation rates are observed in many human genetic disorders," said Dr Anne Goriely of the Weatherall Institute of Molecular Medicine at the University of Oxford, who led the British and Swedish research team.In the case of Apert syndrome, mutations in a gene known as FGFR2 "arise exclusively from the unaffected father and are associated with increased paternal age." The syndrome often results in affected children having distortions of the head and face, and fused fingers and toes - conditions that require multiple corrective surgeries.The team found that the mutation, although harmful to a foetus if the sperm were to fertilise one, appears to be beneficial to the sperm within the cellular environment of the testes. The mutation is preferentially selected even before meiosis, the two-cell division that lead to the production of a sperm cell.The researchers developed a special technique to measure mutation rates in sperm, and found that FGFR2 mutations occur infrequently. They had suspected these might be common events that accumulate with age and become amplified through natural selection.But their relative rareness, and their proliferation - with serious consequences for children who inherit them - seems to be at odds with natural selection as it usually understood.Dr Jim Cummins, a spermatologist at the school of health sciences at Murdoch University in Perth, argues that sperm mutations may be more common than is currently thought, but are rarely harmful: natural selection would weed out only those mutations that reduce or impair fertility."In this case, however, because the kids are badly affected, natural selection would prevent them from passing on the defective gene," Cummins told ABC Science Online.Similar problems are far less likely to occur in women, he said, because their lifetime egg supply - and the process of cell division required to produce eggs - is completed before they are born.But in men, sperm are being made continuously throughout adult life from precursor germ cells that have much more time to be damaged - by exposure to radiation, toxins or disease - in ways that may give rise to mutations."The likelihood of congenital defects arising through faulty cell division in the egg precursors doesn't increase with maternal age," he said. "Of course, advancing age leads to increased random damage to DNA from radiation or chemical exposure, so if this is not repaired, there could be an increased risk of a defect from that source."