Study of color vision in fragile X syndrome.

Various theories have been postulated to account for the unusual inheritance pattern observed in the fragile X syndrome. The recent finding of a secondary amplification of the fragile X mutation in the offspring of carrier females [Oberle et al., 1991; Yu et al., 1991] is consistent with a maternal imprinting process. Laird [1987] has proposed that the fragile X mutation blocks complete reactivation of a previously inactivated fragile X chromosome. We have tested whether or not such a localized block extends as far distal as the red/green color-vision complex at Xq28. We found no evidence of color-vision defects among 25 male subjects with the fragile X syndrome. A fragile X positive woman also had normal color vision, despite being an obligate carrier of her father's gene for red/green color blindness. We conclude that the fragile X gene does not affect the function of neighboring color-vision genes, nor does it affect their ability to compensate adequately for inherited color deficiency on the homologous X chromosome in females.