This finding is not surprising, given the much larger number of cell divisions in the male than in the female germ-line. We would also expect the male mutation rate to increase with the number of cell divisions, and therefore with age, and this is supported by the increaed age of fathers of children with dominant diseases. Moreover, the increase of mutation rate with age appears to be greater than would be expected from the number of spermatogonial cell divisions. Drosophila data argue that the genomic rate of mildly deleterious mutations is at least one per zygote, and very likely considerably more since the data permit only a minimum estimate.
Human data suggest an even higher rate.
H. J. Muller was concerned about the mutational risk to future generations. The rate now appears to be considerably higher than he thought. This raises the question of how the population can rid itself of deleterious mutations without an excessive burden of reduced viability and fertility. The most likely answer, I believe, is quasi-truncation selection.