I don't know this to be fact but this is an explanation a reliable source gave on another board.
"There's nothing GMO about them. They're not golden trout, they're albino rainbows....the history behind them is some fish farmer saw one in his pond that was produced naturally (as natural as a hatchery environment anyway) and then some others and then bred the albinos together....and here we are.
The golden rainbow trout originated from a single rainbow trout that was spawned in the fall of 1954 in West Virginia. This trout's body color was a chimera of golden and normally pigmented tissue. When this fish was crossed with a normally pigmented rainbow trout, the offspring (what we have come to refer to as palomino rainbow trout) were lighter in color.
Golden rainbow trout and palomino rainbow trout are not sterile hybrids, they are simply color variations of rainbow trout (Oncorhynchus mykiss) and should not be confused with the golden trout (Oncorhynchus aguabonita) native to a few drainages in California. It took selective breeding for several generations to result in the development of true breeding golden rainbow trout. Typically, these fish are more of a brilliant golden color than the palomino rainbow trout, which has a color phase intermediate between the golden and normally pigmented rainbow trout.
In Pennsylvania, the rise of the palomino rainbow trout stemmed from obtaining fertilized golden rainbow trout eggs from West Virginia. Subsequently, when these golden rainbow trout reached maturity, they were crossed with normally pigmented rainbow trout and the offspring resulted in the development of the palomino rainbow trout. The initial stockings of palomino rainbow trout in Pennsylvania waters occurred during the 1967 season. At present, however, due to their more brilliant coloration, we use golden rainbow trout exclusively for production purposes rather than the lighter palomino rainbow trout."
