U právního textu bych šla do "biological father", viz také http://en.wikipedia.org/wiki/Father

"Establishment of Natural Father Status
In the event that there is any discrepancy as to the natural father of the child, it is necessary for the male parent to establish his status as the father of the child. Every jurisdiction has laws which specify what is considered a legal, natural father to a child and the methods of proving this relationship. Generally, there are statutory definitions which define the father as including men listed on the birth certificate of the child; men acting in such a way as to hold himself out as the child's father and living with the mother; men who have been judicially determined as the natural father of the child; and finally, men who are described in an affidavit by the child's mother as the natural father of the child."

* viz pokrevní vztah = blood relation

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Note added at 18 min (2012-08-07 11:07:42 GMT)
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Proč tam neuvedli biologický otec-> biological father
ale pokrevní otec -> blood father

Asi neměli strach "se umazat od krve"