es in the array are thought to be responsible for gene expression in the retina, and the severity of color vision deficiencies are therefore roughly related to the differences in the absorption maxima for the photopigments encoded by these first two genes of the array (Deeb and Kohl, 2003, 170-182). A single amino acid difference in the red photopigment can cause a deficiency in normal color vision. Blue cone monochromacy is a rare condition, occurring in roughly 1 in 100,000 individuals, and is caused by the absence of red and green cones from the retina (Deeb and Kohl, 2003, 170-187; Michaelides et al, 2004, 2). This can come about in two ways: either by deletion of a critical gene region which regulates expression of the red-green array; or by mutations that inactivate the red and green pigment genes.
X-linked, recessive, blue cone monochromacy (BCM) is characterized by poor visual acuity, pendular nystagmus, photophobia and photodysphoria (Ayyagari et al, 2004, 75-82; Michaelides et al, 2004, 2). Acuity is usually in the range of 6/24 to 6/60. Photopic vision is mediated by the blue cones, but without a comparison between different classes of photoreceptors, affected individuals have poor color discrimination. Eccentric fixation may be present and myopia is a common finding in these patients. BCM occurs only in males, and is accompanied by severely reduced color discrimination. The disease is caused by point mutations, deletions, and rearrangements of the red and green pigment genes. There are two molecular causes of blue cone monochromatism: the normal L- and M-opsin gene array may be inactivated by a deletion in the LCR located upstream from the L-opsin gene which abolishes transcription of all genes in the opsin gene array, or the LCR is preserved but changes within the L- and M-pigment gene array cause loss of functional pigment production.
The most common genotype in BCM is a single inactivated L/M hybrid g...