Approximately what percentage of blacks have Fy(a-b-) phenotype?

• A. 68%
• B. 5%
• C. 0%
• D. 25%

Answer: (A)

Fy antigens are Duffy blood group antigens on red cells. Fy(a) and Fy(b) are expressed on most people's RBCs, but Fy(a-b-) means there is no expression of either antigen. In many individuals of African descent, a variant in the DARC gene prevents expression of Duffy antigens on erythrocytes, leading to Fy(a-b-). This phenotype is highly prevalent among blacks, estimated at about 68%, so roughly two-thirds have Fy(a-b-). This prevalence is tied to a protective effect against Plasodium vivax malaria, since the parasite uses the Duffy receptor to invade red cells. The other figures reflect different populations (for example, about 5% in Caucasians), so they don’t match the high frequency seen in blacks.