The genetic signatures of people in Spain and Portugal provide new and explicit evidence of the mass conversions of Sephardic Jews and Muslims to Catholicism in the 15th and 16th centuries after Christian armies wrested Spain back from Muslim control, a team of geneticists reports.

Twenty percent of the population of the Iberian Peninsula has Sephardic Jewish ancestry and 11 percent have DNA reflecting Moorish ancestors, the geneticists have found. Historians have debated how many Jews converted and how many chose exile. “One wing grossly underestimates the number of conversions,” said Jane S. Gerber, an expert on Sephardic history at the City University of New York.

The finding bears on two different views of Spanish history, said Jonathan S. Ray, a professor of Jewish studies at Georgetown University. One, proposed by the 20th-century historian Claudio Sánchez-Albornoz, holds that Spanish civilization is Catholic and other influences are foreign; the other sees Spain as having been enriched by drawing from all three of its historical cultures, Catholic, Jewish and Muslim.

The study, based on an analysis of Y chromosomes, was conducted by biologists led by Mark A. Jobling of the University of Leicester in England and Francesc Calafell of the Pompeu Fabra University in Barcelona. They developed a Y chromosome signature for Sephardic men by studying Sephardic Jewish communities in places where Jews migrated after being expelled from Spain in 1492 to 1496. They also characterized the Y chromosomes of the Arab and Berber army that invaded Spain in A.D. 711 from data on people living in Morocco and Western Sahara.