Originally, the only blue egg gene present in the US or Europe came from Araucanas. That is where the blue gene in Crested Cream Legbars came from. Araucana is also the breed that brought blue shell genes to Ameraucanas and eventually EEs.That seems to be mostly true at present, but there's no genetic reason for it to stay that way. From reading hatchery descriptions, I think there are a few working on it, so we may see some true-breeding olive eggers within the next decade or so.
There is no other source of the blue shell gene outside of South America.
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