What are the health risks associated with the MM genotype?
The MM genotype, commonly referred to as a “double merle” or homozygous merle, occurs when a dog inherits two copies of the merle gene. This genetic combination drastically reduces pigment, often producing dogs with primarily white coats, which in turn increases the likelihood of severe congenital health defects.
The most prominent and widely documented health risks associated with the MM genotype are sensory impairments, specifically relating to the eyes and ears:
• Ophthalmic (Visual) Abnormalities: Eye defects are extremely common; one rescue organization reported that nearly 100% of the double merles they take in suffer from some form of eye issue. These dogs have a high risk of being completely or partially blind. Specific ocular conditions include microphthalmia (abnormally small, malformed eyes) and anophthalmia (being born completely without eyes). Additionally, double merles often present with misshapen or off-center pupils, or “star pupils”. When an MM dog has one brown eye and one blue eye, the visual impairments are generally limited to the blue eye, which lacks pigment entirely.
• Auditory (Hearing) Issues: Congenital deafness is another major risk for the MM genotype. This hearing loss can be either unilateral (deafness in one ear) or bilateral (deafness in both ears). One veterinary study found that 25% of double merles were affected by deafness, with 10% unilaterally deaf and 15% bilaterally deaf.
Beyond sensory impairments, inheriting two copies of the merle gene is also associated with a variety of systemic health risks. Double merles have an increased likelihood of suffering from:
• Cardiovascular and cardiac abnormalities.
• Skeletal and reproductive defects.
• An increased risk of developing certain cancers.
Despite this extensive list of potential health defects, not all double merles are entirely blind or deaf. While most will suffer from some level of impairment, many retain enough functional vision or hearing to navigate their environment and go on to live full, happy, and well-adjusted lives.