As in all breeding choices should be made to produce the best results in health, form, type. A hard and fast rule of merle breeding is that a merle factored Chihuahua (whether vibrantly colored or where the pattern is "hidden", such as in a sable or fawn merle) should NEVER be bred to another merle factored Chihuahua as this CAN cause some birth defects. This is not a practice responsible breeders engage in.
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