Final answer:
The probability of a perfect HLA match with a sibling is relatively low due to the high genetic variability of MHC genes. Siblings are more likely to be a good match because they share similar MHC genes.
Step-by-step explanation:
The probability of a perfect HLA match with a sibling is relatively low. The three highly polymorphic MHC I genes (HLA-A, HLA-B, and HLA-C) determine compatibility, and each gene has many alleles within the population. Due to the large number of possible combinations, it is unlikely that a randomly chosen donor will match a recipient's six-allele genotype. However, a sibling has a higher chance of being a good match because they are more likely to have similar MHC genes inherited from their parents.