A complete pedigree of the family studied is shown in . The brother (II-2) of the proband (II-4) was seen, having volunteered for a study on red-green colour vision deficiency. He presented with a classic protanope phenotype (see ). When taking a family history, he mentioned that his father also had a colour vision defect, and that they often disagreed on the appearance of certain objects. Recalling that red-green defects are X-linked, father-to-son transmission is not possible. Further questioning revealed he had a sister (II-4) with colour deficient sons. We mailed copies of the Neitz Test of Colour Vision (Western Psychological Services, Los Angeles, CA) 14
and the AO-HRR (Richmond Products, Albuquerque, NM) 15
, along with instructions for administration. Results revealed two of her sons (III-4, III-6) were protan and the other (III-5) was deutan.
Figure 1 Genetic analysis of a family with two types of colour vision defect. Top, Pedigree of a heterozygous female. I-2, III-1, III-2, and III-7 are all obligate carriers. They had normal colour vision but were untested genetically. I-1 and III-4 (also untested (more ...)
Results from Colour Vision Tests
We then arranged for a subset of the family (II-4, III-5, and III-6) to travel to the lab for detailed color vision testing. The results, shown in , include the Farnsworth Dichotomous Test for Colour Blindness (D-15)(Psychological Corporation, New York), Lanthony’s Desaturated D-15 (Luneau Ophthalmologie, France), the total error score on the Farnsworth-Munsell 100 Hue Test (FM 100)(Macbeth Color and Photometry Division, Baltimore, Md) 16
, the limits of the Rayleigh match range on a Nagel Model 1 Anomaloscope (Schmidt Haensch), the average of 10 settings on the Medmont C-100 17
and the D values (used to quantitatively express the color difference between the symbol and it’s gray background) for the AO-HRR (2002). Neither son was tested on the FM 100, nor was the protanopic son tested on the Medmont C-100. The proband’s (II-4) unique hues (blue, green, yellow) were also measured using a two alternative forced choice paradigm 18
Using a flicker-photometric electroretinogram (ERG), spectral sensitivity was measured over a range of 480–680 nm in 10 nm increments for II-4, III-5, and III-6. In dichromats, this reveals the spectral sensitivity of the single functional cone type in the retina, whereas individuals with both L and M cones, it reflects some combined response of L and M cone function. By fitting such a spectral sensitivity curve to a weighted sum of an L and an M cone spectral sensitivity curve, it is possible to derive an estimate of the relative contribution of the L and M cone populations to the ERG19,20
. This is inferred to represent the relative numerosity of the L and M cones (L:M cone ratio), and evidence to date supports this conclusion 21,22
. Genetic analysis was also performed on II-2, II-4, III-5, and III-6. Here, DNA was extracted from whole blood or buccal swabs as described previously 23
and used in a previously described real-time quantitative polymerase chain reaction (PCR) assay to estimate the relative number of L and M genes in the X-chromosome visual pigment gene array 14
. The L and M genes were selectively amplified by long-distance PCR, and the product obtained was subsequently used to amplify separately exons 2, 3, and 4 of L and of M genes for direct DNA sequence analysis. The primers and thermal cycling parameters for all amplifications were reported previously 20
The mother’s discrimination was tested on a modified version of the Cambridge Colour Test24,25
. The stimuli resemble the plates of a traditional pseudoisochromatic test in which the target and background are made up of many discrete disks that vary in size and luminance. These were generated by a VSG 2/3 video display card (Cambridge Research Systems Ltd., England), presented on a Sony Trinitron 21 inch monitor, and calibrated using a ColorCal colorimeter (Cambridge Research Systems Ltd., England). We modified the test to allow reductions in stimulus size and a greater range of stimulus durations. Chromatic contrast sensitivity was measured along three axes in u
′ colour space. Coloured stimuli were viewed against a background field array of grey dots that had the coordinates of u
′= 0.1970, v
′= 0.4690. Stimulus presentation was 60 msec, and the outer diameter and gap of the ‘C’ subtended 0.68 deg. A staircase method for measuring chromatic contrast threshold was used, where 2 correct responses were required before challenging with a stimulus of lesser vector length. This diminished the impact of guessing on the threshold determination.
Informed consent was obtained from all adult subjects participating in the investigation as well as from parents of participating minors. Research on human subjects followed the tenets of the Declaration of Helsinki and was approved by the Institutional Review Boards at the University of Rochester and the Medical College of Wisconsin.