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Phenotype: Affected dogs typically begin to show vision deficits in low light (night blindness) as early as 6 weeks of age and can be completely blind by 6-8 months. Ophthalmoscopic abnormalities, such as excessively shiny tapetal appearance (tapetal hyperreflectivity), reduced retinal vascularization and pale optic nerve, can be found around 3-4 months of age.
Mode of Inheritance: Autosomal recessive
Alleles: N = Normal, RCD2 = Rod-cone dysplasia type 2
Breeds appropriate for testing: Collie, Smooth Collie, Rough Collie, Old-Time Scotch Collie
Explanation of results:
• Dogs with N/N genotype are expected to not have rod-cone dysplasia type 2 (rcd2). They cannot transmit this RCD2 allele to any of their offspring.
• Dogs with N/RCD2 genotype have one copy of this RCD2 allele and are expected to not have rod-cone dysplasia type 2 (rcd2) but are carriers. They will transmit this RCD2 allele to 50% of their offspring. Matings between two carriers of RCD2 may, on average, produce 25% of puppies with rod-cone dysplasia type 2 (rcd2).
• Dogs with RCD2/RCD2 genotypes are homozygous for this RCD2 allele and will display signs of rod-cone dysplasia type 2 (rcd2).
Rod-cone dysplasia type 2 (rcd2) is a type of early-onset retinal degeneration found in collie breeds. Affected dogs typically display vision deficit in low light (commonly referred to as night blindness) as early as 6 weeks of age. Degeneration of the retina progresses over the next few months, resulting in functional blindness at around 6-8 months of age. Ophthalmoscopic abnormalities such as excessively shiny tapetal appearance (tapetal hyperreflectivity), reduced retinal vascularization, and pale optic nerve, can be seen around 3-4 months of age.
This disorder is caused by an insertion in the retinal degeneration 3 gene (RD3) and inherited as an autosomal recessive trait, meaning that two copies of the RCD2 allele must be present for the dog to develop disease.
Testing recommendations: DNA testing for rod-cone dysplasia type 2 can determine the genetic status of dogs. Dogs with one copy of the RCD2 variant are normal but are carriers. Matings between two carrier dogs may, on average, produce 25% of puppies with rod-cone dysplasia type 2.