Although intentional deception about a child’s paternity can cause legal records and biological paternity to diverge, sexual violence can also be involved, and some legal fathers are fully aware they are not a child’s biological father, but choose to keep that fact secret. How a tree gets tangled Because Y chromosomes are passed unchanged from father to son, descendants of a common male ancestor all share Y chromosomes even 10 or more generations later. When two men whose family trees say they are distantly related through their paternal line don’t share a Y chromosome, it’s proof another man’s DNA is part of the mix—what geneticist Maarten Larmuseau calls an “extra-pair paternity event.”