Reprinted with permission from Biomedical Ethics and Jewish Law, published by KTAV.
Ancient Jewish writings, including the Bible and Talmud, are not devoid of material relating to genetics. One writer describes in some detail how the laws of Mendelian genetics were applied by Jacob in the biblical narrative (Genesis 30:32) of the speckled and spotted sheep (Y. Flicks, “Heredity and Environment,” Techumin, 1982). Hemophilia and its precise genetic transmission is described in the Talmud (Yebamot 64b). The sages in the Talmud and subsequent rabbinic authorities had a remarkable knowledge of the genetics of this sex‑linked disorder [and recognized] that females transmit the disease but do not suffer from it. A few rabbis also considered the possibility of its transmission through males.
Elsewhere (Ketubot 10b), the Talmud portrays a family whose women had hereditary absence of menstruation and no blood of virginity and were obviously childless. The exact nature of the anatomical or physiological abnormality is not described.
It is prohibited in Jewish law to marry a woman from a family of epileptics or lepers (Yebamot 64b; Maimonides’ Mishneh Torah, Issurei Biyah 21:30; Shulhan Aruch, Even Haezer 2:7) lest the illness be genetically transmitted to future generations. According to Rashi (Yebamot 64b), any hereditary disease is included in this category. This talmudic ruling “may well represent the first eugenic enactment, and the only legislative bar to the procreation of a diseased progeny, in ancient and even medieval times (I. Jakobovits, Jewish Medical Ethics).” On the basis of the higher frequency of defective births resulting from union among blood relatives, Rabbi Judah the Pious (died 1217), in his Ethical Will, prohibited marriages between first cousins and between uncles and nieces. Yet such marriages are sanctioned in the Bible and expressly encouraged in the Talmud (Yebamot 62b and Sanhedrin 76b), perhaps to propagate “good genes.” Since consanguineous marriages do not cause birth defects but merely increase their risk, most rabbis do not ban such marriages. Some rabbis, however, strongly caution against it.
Genetic disease was recognized by Maimonides who prescribes a regimen of health for all Jews, since one cannot serve the Lord when one is ill (Mishneh Torah, Deot 4:1). He guarantees anyone who follows his regimen that he will be healthy all his life unless he was born with a hereditary or genetic defect (ibid. 4:20).
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