Any Holstein that ever smelled corn will grade choice.
Shhhh don't tell em
known fact for decades. longhorns will also marble, but their retail cuts are so small, their carcass is only acceptable in niche markets that cater to recovering vegans (those converting back to being an omnivore). it's one of the reasons less muscle means more percent marbling if the marbling percentage stays the same from a regulation point. it's also, if marbling stays the same, but there is more muscle, there will be less percent marbling.
love them or hate them, GAR, through constant selection, will at some point, distill increased marbling while holding muscle steady and eliminate once putative markers. they will do this with the aid of genomics companies for a win win as the company will get better, closer, or linked or whatever markers, while GAR will get the genetics. it will be interesting what the expression distills down to, actual snps, or some other mechanism.