Which of the following phenotypes will react with anti-f?

The correct answer is based on the characteristics of the Rh system, specifically regarding the D antigen. In this context, "anti-f" refers to a type of antibody that reacts with the "f" antigen, which is part of the Rh phenotype that can be expressed when certain combinations of Rh alleles are present.

In the scenario given, the phenotype "rr" signifies that the individual has two little "r" alleles, which do not produce the D antigen, but they can express the f antigen because this antigen is present on the red blood cells stemming from a genotype that lacks large D antigens but still maintains f due to the presence of other associated Rh components. Therefore, when anti-f is introduced, it will react with the cells of an individual with the "rr" phenotype as those cells have the f antigen available for the antibody to bind with.

The other phenotypes, such as R1R1, R2R2, and R1R2, are combinations that produce the D antigen and likely do not express the f antigen in a way that would be recognized by anti-f, leading these to not react with anti-f. Understanding the specific genetic make-up of these phenotypes helps to clarify why the "rr" phenotype