A sibling study of variation in parental mutation rates.

People are born with variable numbers of de novo germline mutations (DNMs), depending primarily on the ages of their parents. To explore additional causes, we developed an approach to call DNMs from nucleotide differences between siblings in genomic regions inherited identical by descent from both parents. Applying it to whole genome sequences from 28,985 sibling pairs of diverse genetic ancestries present in the UK Biobank and All of Us datasets, as well as 2,330 trios, we identified >800K autosomal DNMs and characterized mutation phenotypes in 27,645 sets of parents. We found subtle shifts in the mutation spectrum but no differences in total DNM rates among genetic ancestry groups, or between smokers and non-smokers. Testing for associations between parental mutation phenotypes and their burden of loss-of-function and deleterious missense variants in a set of 180 DNA repair and maintenance genes, we discovered that disruptions in REV1 and LIG1 increase germline mutation rates, and thus that rare mutator alleles segregate in population cohorts.