Genetic basis for donor selection for hematopoietic cell transplantation

The above figure represents the genetic basis for donor selection. Every person has two sets of HLAs (haplotypes), which are located on chromosome 6; one set is derived from the father and one from the mother. These are shown in different colors, reflecting the vast HLA diversity among human subjects.
For any given mother and father there are four equally possible sets of HLA antigens in their children. On average, only one out of every four siblings (25%) will be fully HLA-matched with the patient requiring the transplantation, here shown in the black box. Two out of every four siblings (50%) will be haplotype matches only.
Note that all of the patient's children and both of the patient's parents will only be haplotype matches. It is unlikely that any of the patient's parents, children, or first cousins will be full HLA matches unless the involved families happen to have very common haplotypes or there has been intermarrying among their families.