After hearing of a rise in deteriorating eyesight in a rare breed known as the Swedish Vallhund, researchers traveled many miles, testing hundreds of dogs for their genomes. They narrowed it down to a mutational defect on a gene known as MERTK (c-mer proto-oncogene tyrosine kinase) which causes slow atrophy of the retina. By marking this gene breeders can effectively stop the spread of the mutation.
Another viking breed, the Norwegian Elkhound, was also studied and found to have a mutation on a gene known as the ADAMTS10 gene that cases glaucoma. Again, marking this gene helps breeders identify the mutation and prevent passing it along the bloodlines.