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It is not true, however, for citizenship claims. There must be a blood relationship in order to claim U.S. citizenship. See the U.S. Department of State Foreign Affairs manual for details.

I really doubt that they do DNA to determine parentage.

USCIS, not the referenced manual determines eligibililty. The USCIS allows the naturalization of adopted children. Also, your referenced manual, for what it matters, states that an adopted child is defined as a child of the parent.

SeeSac: I really doubt that they do DNA to determine parentage.

allmendream: A DNA relationship is not necessary for one to be the “legal father”. If Barrack 0bama Sr. is listed on 0bama’s BC; he is 0bama’s “legal father”.

If there is a dispute regarding one's claim to citizenship, a blood relationship is required.

7 FAM 1131.4 Blood Relationship Essential
(7 FAM 1131.4-1 Establishing Blood Relationship)
a. The laws on acquisition of U.S. citizenship through a parent have always contemplated the existence of a blood relationship between the child and the parent(s) through whom the citizenship is claimed. It is not enough that the child is presumed to be the issue of the parents' marriage by the laws of the jurisdiction where the child was born. Absent a blood relationship between the child and the parent on whose citizenship the child's own claim is based, U.S. citizenship is not acquired. The burden of proving a claim to U.S. citizenship, including blood relationship and legal relationship, where applicable, is on the person making such claim.
c. Children born in wedlock are generally presumed to be the issue of that marriage. This presumption is not determinative in citizenship cases, however, because an actual blood relationship to a U.S. citizen parent is required. (Snip)

Yes, Senior is presumed to be Junior's legal father if his name is on the birth certificate. But that legal status wouldn't be the determinative factor in a citizenship claim. If FMD were Junior's biological father, which I am not suggesting is true, Senior's legal status as a parent would be disregarded if Junior could prove a blood relationship to FMD. In U.S. citizenship claims, a blood relationship trumps a legal relationship.