Why is the merle pattern considered grounds for disqualification by the UKC and the Chihuahua Club of America?

Answer

It is believed not to occur naturally in the breed’s DNA

The merle pattern is considered by some established clubs to be grounds for disqualification because it is believed not to occur naturally in the breed’s DNA, suggesting introduction from outside breeds.

Why is the merle pattern considered grounds for disqualification by the UKC and the Chihuahua Club of America?
rareDogtypebreedChihuahua