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HLA (Human Leucocyte A) systema major HISTOCOMPATABILITY complex of four genes (perhaps five) called HLA-A, HLA-B, HLA-C and HLA-D, with the D locus nearest to the centromere on the shorter arm of chromosome 6. Each gene has up to 35 alleles, the particular combination of alleles of the four genes on a chromosome being called the haplotype. Since humans are diploid organisms we have two haplotypes, these being determined by testing the individual's white blood cells for the specific HLA antigens.
Although these large numbers of alleles would appear to make the system a complex one in terms of possible combinations in each individual, in fact the situation is simpler than it seems. The four loci within the HLA system are so tightly linked together that there is very little CROSSING OVER between them. Thus each haplotype tends to be passed onto an offspring as a block rather than as four individual genes. Given no recombination between the four genes, four rearrangements would be expected to occur in the progeny of a mating. Thus any child of a cross has a one-in-four chance of inheriting precisely the same HLA combination as another sibling. Since the success of organ transplantation depends upon a close match of HLA alleles (even one allele difference can cause the foreign organ to be rejected), the chance of matching HLA types is highest within closely related individuals.