Judaism: Ask the Rabbi: DNA Testing
A. The question arose a hundred years ago when blood tests were developed as a means of investigating paternity. The principle behind the testing method was that a person’s blood type was determined by a combination of both parents’ blood.
There is however a Talmudic statement which says that blood is determined by the mother (Niddah 31a). This is accepted by the Shulchan Aruch (Yoreh De’ah 263) and was supported by modern halachic authorities such as Rabbi Benzion Uziel and Rabbi Eliezer Waldenburg, but others such as Rabbi Isaac Halevi Herzog said that rabbis should listen to medical science.
Some recognise that the paternity blood tests are important and useful but are indications without being absolutely decisive. Rabbi Shmuel Auerbach was prepared to sanction DNA testing in a situation in which babies were mixed up in a hospital and it became necessary to know whether baby A could really be the child of a given set of parents.
The tendency these days seems to be that if science and Talmudic positions are in conflict, the scientific position carries weighty status.