Double merle or homozygous merle occurs when the offspring of two merled parents inherit two copies of the dominant merle gene. The odds of this are 25% for each pup born from such a litter. Double merles often have excessive light or white areas and can have hearing and vision problems as a result of having two copies of the merle gene. Homozygous merles can be deaf or blind, or express iris colobomas, retinal detachment, cataracts, persistent pupillary membrane, a displacement of the lens, equatorial staphyloma, night blindness and microphthalmia. Audio impairment or deafness usually develops after the birth of a puppy with their ear canal still closed. The white color of double merles is produced due to the lack of melanocytes which provide high potassium levels in the endolymph surrounding the cochlea's hair cells. There is no surgery or treatment that can reduce the damage. Loss of hearing is directly linked to the amount of pigmentation cells a dog has. The same pigment that is lacking in the ear can also be lacking in the eyes, affecting its development. Although many believe that only dogs with blue eyes have eye problems, it is not correct. Due to the contrast between eye problems and blue eyes, eye conditions in blue-eyed dogs are much easier to spot.
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