So now we're back where we started.
Either a) 7 Jerseys and a Holstein-Friesian got together to produce a great-grand-daughter who phenotypically resembles something very different
or b) Phenotype throwback to something several generations back
or c) someone got a straw of semen mixed up, or mismatched a cow and calf.
I know 'c' happens, I've done it myself. I've no reason to think the farm I got 101 and her herdmates from ever had Brown Swiss, but it could have been in their technician's AB bank to use on another farm.
I usually mate 101 to Brown Swiss because she's one of the few Jerseys in the herd who looks capable of delivering a monsta calf, but I've always had the thought in the back of my mind that that's going to give me a 3/4 calf, not a first cross. No big deal either way... she hasn't held to AI the last two seasons.