As a legal matter, whoever the court would deem to serve the best interest of the child. If the biological father could prove to the court that he is the biological father, he should presumptively be preferred as the person to have custody of the child, but he could be passed over for someone else (either another person who petitions the court for custody or the state, if no one else petitions for custody) if the biological father could be shown to be unfit to exercise custody.
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