The definitions of heirloom and hybrid are discussed in detail on the Prepper Link (PL) forum, so I'll thumbnail it: heirlooms crossed by the same heirloom breed true to the plant that produced them; hybrids bred by the same cultivar/variety or even a flower on the same plant could create a freaky alien Martian plant or might create an even better crop, but it's long odds on that roll of the dice.

Side note: Organic and GMO are totally, completely different topics. Organic is not synonymous with heirloom, nor are all heirloom seeds produced organically.