Final answer:
The property that allows up to 6 different HLAs on the cell surface in a heterozygous individual is termed MHC polygeny and MHC polymorphism, related to the major histocompatibility complex.
Step-by-step explanation:
In a heterozygous individual, up to 6 different human leukocyte antigens (HLAs), encoded by different genes, can be found at the surface of cells. This property is called MHC polygeny and MHC polymorphism. The human major histocompatibility complex (MHC) includes three highly polymorphic genes labeled HLA-A, HLA-B, and HLA-C, which are expressed codominantly. This means that both alleles for a gene are expressed simultaneously, allowing a variety of HLA combinations and making the likelihood of finding an exact match among unrelated individuals quite low. Therefore, a sibling or parent is more likely to be a suitable donor for organ transplants since they are more likely to have a genetic match in the MHC genes.