Figure 88

Genetic principles of the major histocompatibility complex (MHC). The MHC demonstrates a number of genetic principles. Each person has two chromosomes and thus two MHC haplotypes, each inherited from one parent. Because the human leukocyte antigen (HLA) genes are autosomal and codominant, the phenotype represents the combined expression of both haplotypes. Each child receives one chromosome and hence one haplotype from each parent. Because each parent has two different number 6 chromosomes, four different combinations of haplotypes are possible in the offspring. This inheritance pattern is an important factor in finding compatible related donors for transplantation. Thus, an individual has a 25% chance of having an HLA-identical or a completely dissimilar sibling and a 50% chance of having a sibling matched for one haplotype. The genes of the HLA region occasionally (= 1%) demonstrate chromosomal crossover. These recombinations are then transmitted as new haplotypes to the offspring.

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