It's not really a mystery. You just can't buy them. None of them are pure "breeds" in the sense that you mean.Cool!
I know it's still a mystery what the CX "recipe" is, but I wonder if this could help us get closer to finally figuring it out?
They say CX is a 4-way cross.
Since white skin is dominant, all my LegO's have white skin. But Cx have yellow, so both hybrids that make them must have yellow skin.
Would fast growth carry through to the F2 generation? Doubtful, right? So the F1 hybrid parents must be purely fast growing / large sized, respectively.
Therefore, the recipe must involve the 2 mystery breeds being paired as:
Cornish X Mystery 1 (large sized)
Rock X Mystery 2 (fast growing)
And both breeds with yellow skin.
Err... right?
At a certain point, people tried crossing breeds of chickens in various combinations (including Cornish x Rock). Some of the results were so good that they started breeding specific flocks to be the parents of hybrid meat birds.
Over time, they strongly selected for ones that produced the best offspring. They bred for all-white, and they bred out the pea comb from the original Cornish because that gene also causes the skin to be a bit different along the center of the breast, which made the carcass less attractive.
By now, the 4-way cross involves:
Cross two lines to get females that lay lots of eggs. They have hybrid vigor, good egg production to produce lots of eggs for hatching, large eggs so the chicks will be large at hatch, genes for fast growth and lots of meat, sometimes dwarfism so the females will eat less and have fewer size-related health problems. These females have enough genes for fast growth and good size that they need to be raised on a special restricted diet, deliberately stunting their growth (especially muscle growth), to keep the alive and healthy for long enough to be useful as egg producers. They still need restricted feed while laying, which calls for careful balancing: enough to lay but not enough to get oversized.
Cross the other two lines to get males that will mate with those females. The males would also have hybrid vigor, don't need genes for good egg production, but are probably even more extreme than the hens for growing big and fast, so they really need a restricted diet to keep them alive, healthy enough, and small enough to mate successfully. The mating is very important, because the whole point is to produce eggs that hatch, and artificial insemination would be so labor intensive that it's not cost-effective. There would be no dwarfism in the male line, so all the chicks have the dominant trait for not-dwarf.
Crossing those females with those males gives the Cornish Cross chicks that are sold to all of us, or commercially raised for meat.
The "how" is not a secret, but they do not sell the breeding stock. This means that anyone wanting to raise their own can either start from scratch (original pure breeds), or start breeding from Cornish Cross chicks that were meant to be raised for meat. Either way would take so many years that it is not practical for someone else to become a competitor to the current big companies. Some people have tried breeding their own line from Cornish Cross chicks they bought, with various levels of success. Those chicks should have all the right genes to re-create the 4 grandparent lines, but it would take quite a few generations of selective breeding to sort them back into useful true-breeding lines, and most folks are not willing to put in enough time and resources to do that. And of course they require special management at all stages, to keep them from growing to their genetic potential and dying before breeding age.
As an example of how not-secret it is, here's a .pdf on raising the grandparent stock for the Cobb-Vantress line:
https://www.cobb-vantress.com/asset...8454/3450c490-bbd7-11e6-bd5d-55bb08833e29.pdf
The methods are no secret. It's just that you can't buy the birds unless you're a farmer with the right kind of contract with the company, doing things they way they say.