Distribution of adult lactase phenotypes in Bedouins and in urban and agricultural populations of Jordan.

Lactose tolerance tests with breath hydrogen determination identified 39 lactose malabsorbers among 162 Jordanian Bedouins (24%), and 111 lactose malabsorbers among 148 subjects from the urban/agricultural zone of western Jordan and Palestine (75%). This highly significant difference supports the hypothesis that milk dependence in nomadic desert populations resulted in selective pressures in favour of the lactase persistence gene. Within the urban/agricultural zone which extends from the desert border in Jordan to the Mediterranean shore, a significant increase in the frequency of lactose malabsorbers (and hypolactasia gene frequencies) from east to west was observed. The suggested genetic cline is problably due to migration from the desert populations to the agricultural zone.