Researchers estimate that each individual human inherits some 60 genetic mutations not found in either parent—an average of one mutation for every 100 million nucleotides. Previous estimates of human mutation rate have been based on averages of many individuals, but this new count is the first direct measure of the genetic novelties each child brings into the world. The researchers counted the number of new mutations found in two children whose genomes (as well as their parents' genomes) were sequenced as part of the 1000 Genomes Project. The results, published online this week in Nature Genetics, showed that the sources of these mutations vary: while one child derived 92 percent of his mutations from his father's sperm, the other inherited 64 percent from his mother's egg. "This is a surprise," Matt Hurles, senior group leader at the Wellcome Trust Sanger Institute and co-lead author of the study, said in a press release. "Many people expected that in all families most mutations would come from the father, due to the additional number of times that the genome needs to be copied to make a sperm, as opposed to an egg."
Using the methods established in the study, researchers can now examine more families to determine the causes of such variation. It could be that differences in mutation rates result from variation in the accumulation of mutations in the sperm or eggs of each parent or overall differences in each parent's mutations rates. Other remaining questions include the influence of parental age on the number of new mutations passed on to the next generation, and the impact of different environmental factors, such as radiation exposure.