A lot of charolais can throw that pattern. Char in Canada and the USA were bred up to purebred status, which is why you see red, tan, black and grayish purebreds. Most likely that lineback traces back to longhorn. It is a dominant trait, but the homo dilution of the Char breed would hide it (after all, white is white), until you bred a carrier to a non-dilute animal. Chars can also have spots, stars, blazes and other markings, it is just that on a white animal it is kind of hard to see white markings.
The black cow has the spotting gene, and obviously so does the bull she was bred to.
Until I came to Canada I never knew 'snow' was a four letter word. Alberto Manguel