Origins of Hutu, Tutsi and Twa adults has a high capacity for lactose digestion, a surprisingly high number for an agrarian people, which Mamdani suggests may be the result of centuries of intermarriage with Tutsi. Bethwell Ogot in the 1988 UNESCO "General History" further notes that the number of pastoralists in Rwanda increased sharply around the fifteenth century. Although Luis et al. (2004) in a more general study on bi-allelic markers in many African countries found a statistically significant genetic difference between Tutsi and Hutu, the overall difference were not large. Modern-day genetic studies of the Y-chromosome generally indicate that the Tutsi, like the Hutu, are
Origins of Hutu, Tutsi and Twa the importance of the pastoralist migrations. Still other studies point out that cultural transmission can occur without actual human migration. This raises the question of how much of the changes around the fifteenth and sixteenth centuries was the result of an influx of people as opposed to the existing population being exposed to new ideas. Studies that approach the subject of racial purity are among the most controversial. These studies point out that the pastoralist migrants and pre-migration Rwandans lived side by side for centuries and practiced extensive intermarriage. The notion that current Rwandans can claim exclusively Tutsi or Hutu