Merles
Merle is a pattern or marking, not a color per- say. The merle gene dilutes the base color but leaves splotches unaffected there by causing the dog to have a patchy, spotted coloration. Mixed over dark colors you get the "Black or Blue Merles" and over the lighter colors you end up with "Red or Chocolate Merles" There are many variations of colorations, but they usually fall within the broad categories of Black, Blue or Chocolate in Chihuahuas.
A hidden or Phantom merle, is a merle, they are not seen by the color pattern, but will likely have a blue eye or two blue eyes. They should not be bred to a merle. When you are breeding merles you should take pics of the pups when born because there are merles that have spots when born and lose them as they mature. Those are "Hidden" or "Phantom" merles.
Genetically they are merle and can produce merle puppies. Tanzy's sire, Queso, is a “hidden” merle. We know he is a merle because of his beautiful blue eye! He produces merles and solids but majority of his litters come out merles!
One parent has to be merle to get merle. It is a dominate gene not carried ressively. So when people tell you it is carrying the gene they don't understand the genetics. If the puppy has the gene for merle then it will express itself, even if only temporarily. Any non-merle, even if it has a merle parent can safely be bred to a merle, because merle is a dominant gene and is not carried. If you have two non-merles out of merle parents and grandparents and great grandparents, as long as neither is merle they cannot produce a merle because the gene is NOT CARRIED. If you end up with a merle puppy out of two apparent non-merle parents, then one of them has to be a hidden or phantom merle.
Breeding Merles can be tricky. When doubled, the gene can cause defects in puppies such as blindness, deafness, dogs that are sterile, or even stillbirth. This happens when a dog with the Merle gene is bred to another dog with the Merle gene. As long as Merles are bred to Non-Merles there is NO greater chance of genetic defects than with any other color.
Two Merles may produce genetically unhealthy puppies!! Do not breed two merles together!