[4] The need for these registries arose because, in the days prior to DNA parentage testing, the American Quarter Horse Association (AQHA) would not register horses with excessive white markings, sometimes called "cropouts", thinking that such markings were a sign of non-purebred breeding and was maintained for several decades because it was also feared that excess white increased the risk of horses producing a foal with lethal white syndrome (LWS).
This policy arose in part from long observation of the tobiano spotting pattern, which is a dominant gene, and was known to not occur unless one parent is tobiano, a color not recognized in the foundation breeds, such as the Thoroughbred, that were the predecessors of the American Quarter Horse.
What was not understood then is that the overo pattern, found in the Spanish mustang ancestors of the Quarter horse, and sabino pattern, which exists in the Arabian and Thoroughbred, occur as either a gene complex or a recessive gene; thus two solid-colored horses could produce a spotted foal if both were carriers.
It is also known now that lethal white behaves like a recessive, and even two solid-colored horses can carry the LWS gene.
Since the advent of DNA parentage testing and a test for LWS has also been developed, the AQHA has repealed its "white rule" and there are now Paint horses of verifiable Quarter Horse bloodlines that are cross-registered with both the APHA and the AQHA.