Once you move beyond one or two genes to control a trait everything looks a bit like line breeding anyway - you need to have a bunch of genes fixed in a certain arrangement to get the phenotype you're after, and it takes either inbreeding (which line breeding is a form of), breeding lots and lots of shrimp, or both to achieve that.
We're overall in agreement here. My thinking is that there are numerous fixed genes going on in either a cherry shrimp or Okeetee corn snake, by the evidence that breeding to a different phenotype can set you back so much farther than it would if 50% of the offspring would be het for the gene in question if it were a single recessive gene.
However, the reason why I think the color genes are mostly recessive is because it's my understanding that if you cross a colored shrimp with a wild type all the babies are wild type, not an intermediate mix between the two. Is that wrong?
I would say that is incorrect, but with a huge asterisk. I have colors pop up in cull tanks frequently, but that's almost meaningless as evidence to support my hunch. This is where it gets so much more difficult with dwarf shrimp than with a corn snake. There are plenty of wild corn snakes that can be captured and used for breeding that are almost certain to not be heterozygous for the traits you are testing against. And in that hobby, the snake has to either be wild-caught or from a trusted breeder who has never bred it to a color or pattern morph. This is impossible to do in the US with Chinese shrimp simply because any wild type almost certainly came from colored parent stock. And as clear as that wild type shrimp may be, its parents or grandparents almost surely were selectively bred for color and there's a good chance that it still carries the gene(s?). A second complication is that a corn snake can be sexed right out of the egg and never come in contact with its own species before breeding time. In a colony of shrimp, by the time a female is of such a size to determine gender, there's a very real chance she's been bred by a tank mate.
That is a sincere question - this is theoretical to me and I know you have a lot of experience with shrimp. (It might not be the same for every color.) Also, it's my understanding that if you mix Bloody Mary shrimp and Fire Red shrimp you end up with wild type even though both are very red. The fact that these reds don't work together suggests mutations on different genes. Again, I could have bad info and feel free to correct me if that's not right.
It's theoretical to all of us! At least until someone can prove it out by breeding popular color mutations to wild type shrimp that we absolutely know are heterozygous for no mutations. It would also require isolating and growing out baby shrimp alone to test against the wild shrimp, and then doing the same with the next generation of offspring. It wouldn't be *that* hard to do, but would require quite a few aquariums and a source of known wild shrimp that didn't originate from cultivars. But if it were one single simple recessive gene, it could be proven out by the 2nd generation. As for your example, I've heard mixed reports on people breeding BMs to fire reds and so forth, but I don't believe that all wild types are expected. There is also the complication of trade names, and sellers calling bright red shrimp BMs that may not have come from that original line, etc.