The physical problems faced by Charles and the pharaoh Tutankhamun, the son of siblings, point to one possible explanation for the near-universal incest taboo: Overlapping genes can backfire. Siblings share half their genes on average, as do parents and offspring. First cousins' genomes overlap 12.5 percent. Matings between close relatives can raise the danger that harmful recessive genes, especially if combined repeatedly through generations, will match up in the offspring, leading to elevated chances of health or developmental problems—perhaps Tut's partially cleft palate and congenitally deformed foot or Charles's small stature and impotence.