Moonglow is a cross of Roi Charles de Wurtemburg pear x Michigan US 437. Michigan US 437 is a cross of Barseck x Bartlett and Barseck is a cross of BartlettxSeckel. This is all relevant as just mentioned because Potomac is a cross of moonglow x anjou so one might think the fireblight resistance goes back to the original cross of seckel and the same with moonglow and Barseck the FB resistance is from one cross. The key to making these better crosses is Michigan US437 pear. If you are interested in pear breeding you might be interested in this book Fruit Breeding - Google Books