Rough Collies and Smooth Collies have identical standards with the exception of the coat, but interestingly, both varieties can be born in the same litter. As we’re about to wade into the tall grass again, this can only mean one thing: We’re about to talk about the “G” word (genetics), so we’re counting on genetic wonks to help us out if we get in over our heads.
In Collies, the smooth coat is the dominant gene. For a dog to be Smooth, it has to carry the dominant smooth gene.
If a Collie is a rough, however, it doesn’t carry the gene for the smooth coat; Since there’s no such thing as a recessive smooth gene, the only way for to get a Smooth Collie from a Rough Collie is to breed it to a Smooth, and that’s how Rough and Smooth Collie puppies can be born in the same litter.
Roughs cannot produce a Smooth, but the Smooths can produce Roughs since Rough is carried as a recessive gene.