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Why do siblings have different chances of matching for organ transplantation?

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Kidney transplant — transplant British surgeon Andrew Ready flushing a donated kidney during a transplant at Queen Elizabeth Hospital, Birmingham, England, 2006.

Siblings have different chances of matching for organ transplantation because of how the major histocompatibility complex (MHC)—also known as the human leukocyte antigens (HLA) system—is inherited. These antigens, which determine tissue compatibility, are governed by genes located on the sixth chromosome.12

Because individuals inherit one of each pair of chromosomes from each parent, the inheritance of these antigens follows Mendelian laws of inheritance. When parents pass their chromosomes to their children, siblings have the following statistical probabilities of matching:

1-in-4 (25%) chance: A complete match, where siblings receive the same paternal and maternal chromosomes.12
1-in-2 (50%) chance: A half-match.1
1-in-4 (25%) chance: No match at all.1

While identical twins share the same antigens and are perfect matches, the high degree of polymorphism in HLA genes makes it extremely unlikely for non-sibling individuals to share the same tissue type. Tissue typing is used to assess these matches to help minimize the risk of graft rejection.132