How was ethnicity of Hutu and Tutsi socially constructed by the Belgians?

In 1916 Belgians took control over the area as a colony.
 Prior to this the Hutu and Tutsi were were both living in two kingdoms in the
region, Ruanda and Urundi, with both being ruled by the same king. While they identified
as different groups, intermarriage was common and there was little animosity between the
groups.  When the Belgians colonized the two kingdoms they chose the Tutsi, who
comprised about 14% of the population, to be the ruling class.  They were chosen both
because this gave the Belgians a proxy to act for them in the region, and because the
Belgians felt that the Tutsi were biologically superior to the
Hutu.

It was therefore the Belgians who socially
constructed the ethnicity of these two groups into what they became post-colonization.
The labels imposed on the Tutsi for being superior caused the Hutu (who always made up
the majority of the population in both the precolonial kingdoms and
the Belgian colony)to become second class citizens, which they then rebelled against.
This rebellion is what led to
the Rwandan genocide.

Purvis, A. (1996). Roots
of genocide. Time, 148(7), 57. Retrieved from
Academic Search Premier database.