Celiac Disease (CD) Is a Common, Usually Inherited Human Disorder

Celiac disease (CD) is a common, usually inherited human disorder of intestinal inflammation that is triggered by eating gluten, a major protein in wheat and other cereal grains. It is the most frequent food intolerance. One of the three genes involved encodes SH2B3, a protein involved in innate, nonspecific immune response to bacterial pathogens. Celiac sufferers frequently have a particular allele of this gene, an allele so common in the human population that which of the following might be hypothesized?

A) The allele must have been positively selected for during a period of especially frequent bacterial infections.
B) The gene must code for a T- cell receptor for a common bacterial component.
C) The non- celiac allele must have been selected against in previous human populations.
D) The gene must be common to all primate genomes.
E) The gene must be common in most mammalian genomes.