Final answer:
Weak D point mutations primarily occur on the portion of the protein that is external to the cell membrane, affecting the RhD antigen's structure on red blood cells and its antigenicity.
Step-by-step explanation:
Weak D point mutations primarily occur in the RHCE gene that encodes the RhD erythrocyte membrane protein. These mutations usually occur in the regions of the gene that affect the protein's structure on the surface of red blood cells, namely only on the portion of the protein that is external to the cell membrane. This region is exposed to the immune system, and mutations here can lead to variations in the RhD antigen which may reduce its antigenicity, leading to weak D phenotypes.
Mutations that occur on the portions that are intracellular, in the membrane itself, or only on the intramembrane part, can also affect protein function. However, weak D point mutations specifically refer to mutations affecting the extracellular part of the RhD protein that weaken its antigenic properties but still enable it to be present, potentially causing discrepancies in RhD typing.