Hold my beer; I'm gonna middlebrow this! My best guess (dimly remembered from drawing blood for testing in my lab) is that these "groups" (systems?) all live at

Hold my beer; I'm gonna middlebrow this! My best guess (dimly remembered from drawing blood for testing in my lab) is that these "groups" (systems?) all live at the same place on the chromosomes that do/n't express them — they're alleles.