Rhode Island Whites have rose combs and hatchery Red Sex Links have single combs so they aren’t using Rhode Island Whites. I believe they use Delaware hens.Ladies and gentlemen, my friends did the experiment using RIR roosters over RIW hens and over white rocks (WPR).
The daughters who had RIW as mothers had all the common coloration we see in the 90% of the commercial sex links of all the companies.
The daughters who had WPR as mothers didn't have any necklace (the neck was totally brownish feathered) and their tail was black!
The mother of the 90% of the commercial red sex links is RIW of course selected for smaller size and smaller appetite.
You can see the official sites of dominant CZ and of cackle hatchery to check for multiple types of red sex links.