Merle is a dominant gene which does not occur naturally in the Chihuahua. So for a merle gene to show up within a DNA test, it means that somewhere in its pedigree it has been cross bred.
In Australia, the United Kingdom and New Zealand a Chihuahua who expresses the merle gene, cannot be registered as a purebred, as it cannot be anything but a crossbreed.
As a very basic statement. Merle is a problem in how it works to express colour as it lightens and whitens, which affects the pigment inside the ears and eyes. This can lead to deafness and blindness as the nerve endings atrophy and die off.
According to Idexx, (global leader in pet healthcare) health problems associated with the merle allele both heterozygous merle (Mm) and homozygous double merle (MM) may exhibit auditory and ophthalmic abnormalities including mild to severe deafness, increased intra ocular pressure, ametropia, microphthalmia and colobomas. The double merle genotype may also be associated with abnormalities of skeletal, cardiac and reproductive systems.
Heterozygous – means one parent only is merle.
Homozygous – means both parents are merle.
Ophthalmic – relating to the eye and its diseases.
Occular pressure – is a build up of fluid that flows throughout the inside of the eye.
Ametrophia – is a state where refractive error is present or when distant points are no longer focused properly to the retina.
Microphthalmia – is where the one or both eyes don’t develop properly.
Colobomos – when part of the tissue that makes up the eye is missing.