2. In 1950, geneticist Bentley Glass studied a population of over 200 Dunkers in southern Pennsylvania. Glass used the MN blood group, a blood type controlled by a single gene with two loci. Individuals may be type M (homozygous for the M allele), N (homozygous for the N allele), or MN (heterozygous). The MN blood type has little clinical significance, and as far as is known there is no survival advantage in having one MN blood type over the other.