PDC and Abnormal retinal morphology: Seventeen of these were chosen for their proximity to 14 loci known to be involved in dog or human retinal diseases, or being expressed in the retina: ABCA4, CNGA1, GNAT1, GNGT1, NRL, PDC, PDE6A, PDE6D, PDEG, PRCD, RDS, RLBP1, ROM1 and SAG. The rest of the markers were selected to cover the whole of the autosomes at intervals of approximately 20 cM.