What does the genotype —/— represent in the Rh system?

In the Rh system, the genotype —/— is indicative of Rhnull. This genotype means that an individual lacks all forms of the Rh antigen on their red blood cells, which includes the D antigen—this is a critical aspect of the Rh blood group. Rhnull individuals are very rare and can experience significant challenges during blood transfusions since they can only receive blood from other Rhnull individuals due to the absence of these antigens.

The presence of both dashes (—/—) signifies a complete absence of the Rh antigens rather than just being negative for the D antigen or being a D mosaic, which would involve the presence of some D antigen alleles. Additionally, the term "Total Rh" is not a standard term recognized in the context; hence it does not accurately describe the genotype in question.

Understanding the implications of this genotype is crucial for transfusion medicine and patient management, as Rhnull patients are often considered "universal recipients" in the context of Rh blood group compatibility but require very specific blood types for safe transfusion.