This condition is known by many names; day blindness, hemeralopia and cone degeneration - to name a few. These names all describe the same condition, a hereditary visual problem related to intense light.
     Within the retina are rod and cone cells. These cells are light sensitive (photo-receptor) and become stimulated by light. The stimulated cells send electrical impulses (via the optic nerve) which are translated by the brain into "sight". Each of the receptor cells serve specialized functions. The rod receptors function in limited (dark) light enabling the dog to have night vision. The cone cells function in intense (bright) light giving the dog its day sight.
     Day blindness is the result of the degeneration of the cone receptors. Without the functioning of the cone cells, to translate the bright light the dog cannot  see in  bright conditions. The rods, having been designed for functioning under dim light, overload in intense light - similar to us looking at a camera flash bulb. Cone cell degeneration is genetically controlled. It is an autosomal recessive characteristic. This means that there are only 3 genetic possibilities: Clear, Carrier, or Recessive.
     Affected pups often show indications of this condition as early as 8 weeks of age. The condition may be suspected by close observation of the pup's behavior. The following symptoms may be indicative of day blindness: bumping and stumbling over objects as though the pup never saw that the object was there, going into bright sunlight - does the pup appear to be feeling its steps, is the dog constantly seeking shaded areas, is the dog very hesitant to be coaxed into daylight, does the dog's reactions improved drastically in the evening or under dim light ?
     A present dogs can be tested by means of an Electro-Retinogram (ERG), which is performed under general anesthesia, in which a light is projected in front of the subject's eyes, while sensors measure the electrical impulses generated by the rods and cones. By varying the stimulation input, rod and cone output function can be determined separately. This test enables the owner to have a positive identification of the problem. The short coming of the test is that it can only be affective in identifying dogs that have cone degeneration. Dogs that are clear of the disorder and dogs that are carriers will both have good results from the ERG test.
     The difficulty in ridding the gene pool of the Cone Degeneration gene, lies in determining which dogs are carriers.  Carrier dogs will appear normal, but can pass the fault on to their offspring. By test breeding dogs their genetic type can be determined (clear or carrier). Anytime a day blind puppy is produced, both parents must be carriers of the gene. By testing all puppies, removing all carriers from breeding programs and never breeding recessive, day blind dogs - this gene can be eliminated from the gene pool.