In the HLA nomenclature, the convention is to show first the locus, followed by an asterisk and a four-digit number. The first two digits are the allele number, and the last two digits indicate a unique sequence for variants. Only unique sequences fit within the new nomenclature. It is now known that there are four DRB1 chains for DR2 . Alleles Two terms, haplotype and linkage disequilibrium, describe important associations between MHC genes.
Haplotype refers to the set of genes on any one chromosome.
Every individual has two haplotypes of the MHC, one from each parent. The second term, linkage disequilibrium, refers to the observation that some HLA antigens coincide within a single haplotype much more frequently than expected. If discrete genes were distributed independently throughout the population, the frequency at which any two linked antigens encoded at different loci would occur within a haplotype would simply be the product of their frequency in the population. Such linkage disequilibrium may occur because not enough evolutionary time has elapsed for the genes governing the antigens to be normally distributed or because such an association results in a selective advantage to the individual. Over many generations, such recombination leads to an equilibrium of linked alleles in a population unless selective pressures favor survival of certain haplotypes.