Let's say both of someone's parents were half Irish, half German. Could you get kids that were fully Irish or German? No, they would still be half German, half Irish. But this doesn't lend to chickens, because they are chicken breeds, not chicken ancestry. They are bred specifically for certain looks and traits.
Here's the thing. DNA doesn't determine a breed. And with chickens, pedigree doesn't either. The phenotype (how it looks) and how it breeds true does. So if for whatever reason you get a bird that meets the Ameraucana Standard, not just leg colors, blue eggs, beard, and comb but also type and coloring, technically it is an Ameraucana. But only if this breeds true. If you bred this "Ameraucana" with other Ameraucanas and you got Ameraucana chicks who represented their breed and variety, technically it's an Ameraucana. The first Ameraucana varieties were developed from Easter Eggers and new Ameraucana varieties are still developed from Easter Eggers.