How likely are siblings to be a complete match for human leukocyte antigens?

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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.

In the context of transplant surgery, siblings have a one-in-four (25 percent) chance of being a complete match for human leukocyte antigens (HLA).

According to the laws of Mendelian inheritance, siblings also have a one-in-four chance of having no HLA match and a one-in-two (50 percent) chance of being a half-match. This matching process is critical because tissue typing helps minimize the risk of graft-versus-host disease, a dangerous immune reaction that can occur when donor and recipient tissues are mismatched.12