That difference is actually a survival benefit: The combination of two individuals' different MHC (major histocompatibility locus) genes gives their offspring an advantage in beating back disease ...

When these two related strains are intercrossed to elucidate the chromosomal locations of diabetes susceptibility genes, homozygous expression of the NOD MHC alleles is essential for disease ...

Although elegant, this model was not verified. Hybrid resistance was mapped to several putative Hh loci located close to MHC class I genes, but no corresponding genes or antigens were ever found.