The Red-Light Absolute Threshold in Heterozygote Protan Carriers

Possible Genetic Implications

¹ Foundation for Ophthalmic Research, University of Chicago, and Department of Medicine, City of Hope Medical Center, Duarte, Calif.

The absolute red threshold was evaluated in 8 protan carriers and 5 protans from five families. Identification of the color defects was made from tests on the Nagel anomaloscope. Numerous retinal areas were studied in an attempt to test the recent X chromosome inactivation hypothesis of sex-linked inheritance.

A significant elevation of the absolute red-light threshold was shown at a 5 degree area in all 3 protanopes and 5 carriers of this trait tested. The same elevation was noted in 2 of 3 protanomals and in 2 of the 3 carriers of this trait tested. The areal pattern of the abnormality was similar in involved males and females. The degree of threshold elevation at 5 degrees could not be related to the severity of chromaticity defect shown by a subject. Elevations were of the same order in protanopes, protanomals, and their female relatives.

This dissociation of brightness and chromaticity function in protans was postulated, to occur because of rods substituting for cones as red-light detectors. Consequently, chromaticity measurements were said to reflect cone function, and red-light brightness measurements rod function.

The findings in this study did not conclusively substantiate or reject the X chromosome inactivation hypothesis, possibly because of the parameter tested. However, the usual minimal chromaticity abnormalities in heterozygote carriers may be explained by a pattern of mosaicism.