Figure 1. Study pedigree of dogs segregating cone-rod dystrophy

Two affected pit bull terriers (animals 1 and 3) were outcrossed to homozygous normal animals (N1-3). Heterozygous offspring were then either intercrossed or bred to affected animals to generate informative litters. Electroretinography was used to determine the presence of the disease. Affected animals are denoted by solid symbols while asymptomatic animals are illustrated either with half filled (where obligate heterozygotes) or empty symbols (where both parents are non-affected and status at the mutation is unknown). Complementation test-breedings resulted in animals 29-37, and 150-151, all ten of which were non-affected, establishing that the diseases derived from animals 1 and 3, although phenotypically similar, are non-allelic.