Cone Degeneration, Achromatopsia

Segregation of the domestic dog population into separate breeds, driven by artificial selection for defined phenotypic and behavioral traits, has been marked by the emergence of inherited canine diseases that are often directly comparable to those observed in humans. In humans, a condition referred to as achromatopsia, total color blindness, day-blindness or rod monochromacy has been extensively described [4].

Cone degeneration (CD) is an autosomal recessive canine disease that occurs naturally in the Alaskan Malamute , Miniature Poodle and German Shorthaired Pointer breeds, although this condition may exist in other breeds of dogs [3]. Canine cone degeneration (cd ) was first observed in an inbred strain of Alaskan Malamute dogs in 1960, and is inherited as an autosomal recessive trait (15). Cone-degenerate pups develop day-blindness and photophobia between 8 and 12 weeks postnatal, the age when retinal development is normally completed in dogs. Symptoms are present only in bright light; vision in dim light is normal. Affected dogs remain ophthalmoscopically normal throughout life.[4] It is similar to human achromatopsia, a heterogeneous autosomal recessive disorder. Both the canine disease and its human counterparts are characterized by day-blindness and absence of retinal cone function in adults [2]. You will know that your dog has day-blindness when you notice him colliding with obstacles in daylight.

Cones are light-sensitive structures in the vertebrate eye. They are found throughout retina and are concerned with discrimination of color and visual acuity. There are three types of cone cells each containing a different iodopsin and each giving maximum response when stimulated by the blue, green and red. A pigment defect in one or more of the types of cone cells can lead to color blindness.

In humans, achromatopsia is characterized by reduced visual acuity, pendular nystagmus, increased sensitivity to light (photophobia), day blindness, reduced or complete loss of color discrimination. All individuals with achromatopsia (achromats) have impaired color discrimination along all three axes of color vision.

Achromatopsia is inherited in an autosomal recessive manner. At conception, each sibling of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Once an at-risk sib is known to be unaffected, the risk of his/her being a carrier is 2/3. Heterozygotes (carriers) have no symptoms. Carrier testing for family members at risk for CNGA3 or CNGB3 mutations is available on a clinical basis once the mutations have been identified [1].

Autosomal Recessive Traits are hereditary traits carried by genes that are expressed only when an individual has two copies of the gene. A human or animal needs to inherit the affected gene from both parents for the genetic trait to be expressed. When a disease is inherited as an autosomal recessive trait, the parents do not usually have the disease themselves but are symptomless carriers. Examples of autosomal recessive disorders include sickle cell anemia and albinism.

Phenotype is observable features of an individual organism that result from an interaction with the environment in which development occurs. Variations due to nature are the inherited aspects of the organism, the genotype.

References: REF FILE #126

Animal Calendars