Mutation timing, accumulation, and selection in the male germline shape inheritance risk for developmental disorders.

De novo mutations (DNMs) in the paternal germline are a major cause of developmental disorders, but how mutation timing, paternal age, and spermatogonial selection jointly shape transmissible risk within individual fathers is unclear. We combined trio whole-genome sequencing from 167 families with deep targeted NanoSeq profiling of sperm from 127 fathers of children with confirmed pathogenic DNMs. Transmitted DNM burden and paternal sperm mutation burden, spectra, and selection landscape were indistinguishable from population reference cohorts. Six fathers carried pathogenic early mosaic variants detectable in sperm at variant allele fractions (VAFs) of 0.7%-14.8%, creating individual recurrence-risk outliers. However, early mosaics accounted for ∼8% of the cohort-aggregated pathogenic burden exome-wide, compared with ∼18% from known positively selected drivers and ∼74% from other rare variants accumulating with paternal age. Thus, paternal de novo disease risk is shaped primarily by universal age-associated mutation and selection, while early mosaicism creates uncommon but clinically important high-risk individuals.