My understanding (from another board that features the posts of a poultry geneticist) is that the Hyline "patented" birds most likely do have some Leghorn ancestry in there somewhere, although it's not a simple first-generation cross. Most of their birds are not merely a simple cross. They are a "secret recipe"
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However, Old Rando is right that most of the hatchery sex-links you buy don't have Leghorn ancestry. They are usually Sil-go links of one type or another, using RIRs and RIWs or White Rocks or the like.

However, Old Rando is right that most of the hatchery sex-links you buy don't have Leghorn ancestry. They are usually Sil-go links of one type or another, using RIRs and RIWs or White Rocks or the like.