Where do spontaneous mutations come from?

Who is James F. Crow? 1997 The high spontaneous mutation rate: Is it a health risk?
James F. Crow

"How can we account for a higher mutation rate in males than in females? The most obvious explanation lies in the much greater number of cell divisions in the male germ line than in the female germ line. In the female the germ cell divisions stop by the time of birth and meiosis is completed only when an egg matures. In the male, cell divisions are continuous and many divisions have occurred before a sperm is produced. If mutation is associated with cell division, as if mutations were replication errors, we should expect a much higher mutation rate in males than in females."This makes the strong prediction that the mutation rate should increase with the age of the father, since the older the man, the more cell divisions have occurred. On the other hand, there should be no age effect in females."

"The most important properties of gene mutations, for the purposes of this talk, are: First, to repeat, if they have an observable effect they are almost always harmful. Second, most of the changes are not in the genes, but in the great bulk of so-called "junk" DNA, most of which has no known function. Many of these changes are effectively neutral. Third, most mutations have very minor effects, if any. We usually think of a mutation as an eye color change, a conspicuous disease, or some other phenotypic change that is sharp and striking, and indeed these are the kinds of mutations that have been most useful for classical genetic analysis. But diverse experiments in various species, especially Drosophila, show that the typical mutation is very mild. It usually has no overt effect, but shows up as a small decrease in viability or fertility, usually detected only statistically. Fourth, that the effect may be minor does not mean that it is unimportant. A dominant mutation producing a very large effect, perhaps lethal, affects only a small number of individuals before it is eliminated from the population by death or failure to reproduce. If it has a mild effect, it persists longer and affects a correspondingly greater number. So, because they are more numerous, mild mutations in the long run can have as great an effect on fitness as drastic ones."


"I conclude that for a number of diseases the mutation rate increases with age and at a rate much faster than linear. This suggests that the greatest mutational health hazard in the human population at present is fertile old males. If males reproduced shortly after puberty (or the equivalent result were attained by early collection of sperm and cold storage for later use) the mutation rate could be greatly reduced. (I am not advocating this. For one thing, until many more diseases are studied, the generality of the conclusion is not established. Furthermore, one does not lightly suggest such socially disruptive procedures, even if there were a well-established health benefit.)"

Why Do Some Mutations Not Show a Paternal Age Effect?