Reciprocal recombination and gene conversion (in short, exchanges) have undoubtedly been playing important roles in molecular evolutionary processes. Such exchanges may be revealed by comparing DNA sequences. However, it is not easy to obtain clear-cut evidence for the occurrence of exchanges between orthologous regions of DNA, because alleles at a functional locus generally differ very little [see for human nucleotide differences at 49 loci]. Demonstrable instances often involve exchanges between paralogous loci [see for a review of evolution of globin gene families].
The major histocompatibility complex (Mhc) loci provide an exceptional opportunity for studying sequence exchanges. Mhc molecules present processed foreign peptides to T-cell receptors (Tcr). In order to bind varieties of such peptides and to initiate the immune response as early as possible when infected Mhc genes must diversify in a nonclonal fashion within a population of species rather than at the individual level . The extent of Mhc polymorphism is extraordinary in terms of the number of alleles and the number of nucleotide differences between alleles (Klein 1986). Therefore, there is a good chance of demonstrating sequence exchanges at the Mhc loci, and it is not surprising that there have been a number of attempts at such demonstrations (