Blue-cone monochromats , who concluded that they had rods and S-cones, but lacked M- and L-cones. Though two studies suggested that blue-cone monochromats might also possess a second cone type containing the rod photopigment Pedigree studies show that blue-cone monochromacy is an X-linked recessive trait (e.g., Falls, 1960; Spivey, 1965) . A molecular genetic analysis of the M and L-cone photopigment gene array on the X-chromosome of blue-cone monochromats shows that the deficit can arise for a number of different reasons, including deletions, or loss of function due to homologous recombination and point mutation (Nathans et al., 1989; Nathans et al., 1993) Spectral sensitivities in "classic" blue-cone monochromats have been measured several times before ( , and are typical of the S-cones. A concern about the use of blue-cone monochromats to obtain a standard S-cone spectral sensitivity for central vision, however, is that they usually fixate extrafoveally (but there are exceptions, see Hess et al., 1989). Consequently, in order to use blue-cone monochromats to estimate normal S-cone spectral sensitivity, it is necessary to estimate their macular and photopigment optical densities, and, if necessary, correct them to normal density values. Moreover, some individuals in pedigrees with blue-cone monochromacy reveal residual L-cone function if large or even small test fields are used | |
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