Final answer:
The correct statement about the Duffy blood group system is that Fya and Fyb are codominant alleles, where both antigens are equally expressed when inherited from the parents.
Step-by-step explanation:
The Duffy blood group system includes antigens that are located on the surface of red blood cells. One of the true statements about this system is that Fya and Fyb are codominant alleles. This means that if a person inherits the Fya allele from one parent and the Fyb allele from another, both antigens will be equally expressed on the red blood cells. The majority of whites are actually Fy(a+b-), not Fy(a-b-) as suggested in the question. Enzymes do not enhance the Duffy antigens; rather, the presence of these antigens is due to genetic expression. Antibodies can show dosage effects, meaning that the strength of the antigen-antibody reaction can vary based on the concentration of underlying antigenic substances. Therefore, option 3 is correct: Fya and Fyb are codominant alleles.