This paper considers the social and infrastructural contexts leading to the discovery of a specific inherited condition, deficiency of the enzyme glucose-6-phosphate dehydrogenase (G6PD), in certain populations within Iran and Israel during the 1950s and 60s. G6PD deficiency, which occurs with high frequency among Middle Eastern, South Asian, and Central African populations, causes affected individuals to have severe hemolytic reactions to certain antimalarial medications, infections, or foods, notably fava beans, resulting in the condition’s popular name of “favism.” The prevalence of the condition among populations living in malaria-endemic areas has been a major concern for national and international programs to prevent and treat malaria outbreaks. Accordingly, medical scientists in Israel and Iran, in collaboration with visiting American and British researchers, developed diagnostic tests and conducted field surveys to manage and raise awareness about favism. However, favism quickly took on not only clinical but also anthropological significance, because its frequency often varies dramatically between groups divided by social factors such as religion, language, or tribal affiliation. Using scientific publications, personal correspondence, and oral histories, I argue that medical research on G6PD deficiency created an international scientific discourse that perpetuated local nationalist visions of ethnic identity and history. In Iran, physicians found that the Persian-speaking Shi’ite Muslim majority carried the gene, while Zoroastrians and Armenians lacked it. Drawing on Pahlavi-era nationalist historiographies that cast Zoroastrians as the last living representatives of the “original” gene pool of the ancient Persians prior to the Arab-Muslim conquests, Iranian and American geneticists alike argued that the original inhabitants of Iran lacked any mutation for G6PD deficiency until foreign invaders introduced it. This medical narrative corresponded well to Pahlavi sociocultural attitudes that perceived the Arab-Muslim invasions as marking the beginning of Iran’s civilizational decline. Meanwhile, Israeli scientists, who overwhelmingly belonged to the new state’s Ashkenazi sociopolitical elite, identified G6PD deficiency as an ethnic condition occurring only in “non-Ashkenazi” Jews, i.e. marginalized Jewish immigrants from the Mediterranean and Middle Eastern diaspora. These Ashkenazi researchers disagreed with one another, and with European and American geneticists, over how to reconcile this pattern of biological difference between different Jewish groups with a Zionist model of history in which all Jews shared an original gene pool in ancient Judaea. Ultimately, the production and circulation of knowledge about G6PD deficiency illuminate the complex power dynamics between a Western-dominated global biomedical community, Middle Eastern scientists, and disenfranchised research subjects.