The issue of DNA testing to determine paternity has been discussed extensively in recent halachic literature. See Encyclopedia of Jewish Medical Ethics, by Dr. Avraham Steinberg, "Paternity," vol. III, pp. 775-782; Refuah, Metzius v'Halachah, by Rabbi Dr. Mordechai Halperin, ch. 32-33, pp. 383-418; and Gray Matter, by Rabbi Chaim Jachter, "Blood Tests and DNA in Beit Din," vol. III, pp. 168-181.
In short, according to many authorities, a mismatch of DNA testing can serve as almost absolute proof to disprove paternity, such as to exempt a supposed father from providing financial support to a supposed child and to disqualify him from inheriting. However, according to several authorities, it falls short of the certainty required to declare the child a mamzer.
On the other hand, a match of DNA testing serves only as a statistical majority (rov) to establish paternity, and cannot obligate a man to provide financial support for a child not presumed to be his. However, it can be used, in certain circumstances, to allow an agunah to remarry. (See Techumin, vol. XXI, pp. 1221-123 and Kol Zvi, published by RIETS Kollel Elyon, 5762.)
In the case of a shetuki, the use of DNA testing to verify that a potential spouse is not a sister is usually superfluous. If the mother is able to vouch for the validity of the child as a legitimate one, he is permitted to marry a Jewish woman without concern that she is his sister, regardless. (See Rambam, Hil. Issurei Biah 15:29; Beis Shmuel E.H. 4:54,66.) If the mother is not able to vouch, the Sages imposed a concern to the legitimacy of his father and in certain cases the shetuki is limited to marrying a convert. In most situations nowadays, though, upon turning to Beit Din he will be permitted to marry a Jewish woman (E.H. 4:36; 6:18; Pischei Teshuvah 4:41). Presumably, he would also be able to marry a Jewish woman without concern that she is his sister.
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