For decades, mothers have received disproportionate attention for the mutations their children inherit. Down’s syndrome is widely known to result from “aging” eggs, and many X-linked disorders appear in males, passed down by their mothers. But now, increasingly sophisticated genetic analysis reveals the role of aging fathers. Nature reports a study of a large Icelandic family–Iceland is known for a relatively isolated gene pool, and for widespread cooperation with genetic studies. This study shows that, whereas the mutation rate in women is fairly constant, the father’s sperm mutation rate increases exponentially–doubling every 16.5 years.
This finding explains some old mysteries, such as why the X chromosome that passes on hemophilia comes more often from the mother’s father than from the mother’s mother. Of course, it had to be a late mutation (in the sperm) because otherwise the mother’s father would have had hemophilia.
Why would the father’s sperm mutation rate increase so fast? The basis is unknown, but it may have to do with the high number of generations experienced by sperm cells, which produce new sperm continually throughout life, unlike human egg development, which begins before birth and involves fewer cell divisions.
How might the paternal mutation rate explain the rising incidence of mental disorders such as autism and schizophrenia? The brain is the most complex part of the body, estimated to require function of a quarter of the genes in the human genome. Many of these genes function in multiple parts of the body; but if one such gene is defective, the entire brain is impaired. That explains why so many inherited disorders include mental deficiency as part of the syndrome (see OMIM for examples). So a rising mutation rate with paternal age is particularly likely to knock loose something in the child’s brain.
Speculating again, I can’t help wondering if this explains why primate females actually prefer marginal young males to the older “alpha males” they were thought to favor. DNA paternity studies of chimpanzees and baboons largely upended the old alpha-fathers-all model, replacing it with a much more complex view of female mate choice.