From the following phenotype Anti-D 0, Anti-C 0, Anti-E 0, Anti-c +, Anti-e +, what is the most common genotype?

• A. rr
• B. R^1r
• C. dCE/dCE
• D. CDE/cde

Answer: (A)

In the Rh system, D antigen is determined by the RHD gene, while C/c and E/e are determined by RHCE. The given phenotype shows no D, no C, and no E antigens (D-, C-, E-), with c and e present (c+, e+). To be D-, an individual must have two nonfunctional RHD alleles, i.e., rr. The other genotypes listed would produce D antigen or C/E antigens that contradict the observed phenotype (for example, having D antigen with R^1r or C/E antigens with dCE/dCE or CDE/cde). Therefore rr best fits the observed serology. (Note: the full genotype behind this phenotype would be rr with ce/ce RHCE alleles, but among the options, rr is the correct choice.)