Blue-cone monochromacy

Blue cone monochromacy is an inherited eye condition tied to the X chromosome and passed on in a recessive pattern, meaning it almost exclusively affects males. The disorder stems from a lack of functional red-sensitive (L) and green-sensitive (M) cone cells in the retina, which leads to severe color blindness, poor visual acuity, involuntary eye movements (nystagmus), sensitivity to bright light (photophobia), and hemeralopia—difficulty seeing in daylight. Visual acuity typically falls between 20/60 and 20/200. Symptoms are usually stable, though some research hints at possible progression over time.

Color discrimination is profoundly impaired in blue cone monochromats. However, in twilight conditions, the interaction between blue cones and rod cells may allow a limited form of dichromatic vision. The condition arises because only the short-wave-sensitive (S) cones, which detect blue light, and the rods, responsible for night vision, remain functional. The red and green cones, which normally cluster in the fovea and drive sharp central vision, are missing or defective.

Genetically, BCM involves a cluster of three genes at position Xq28 on the X chromosome. Two of these genes encode opsins for red and green light, which are 96% similar due to a recent duplication event, differing at only 19 amino acid sites—seven of which tune their spectral sensitivity. These opsin genes share only about 40% similarity with the blue opsin gene (OPN1SW) on chromosome 7 and with rhodopsin (RHO) on chromosome 3, both of which remain unaffected in BCM.

Mutations that cause BCM effectively combine the genetic defects of protanopia (no functional L-cones) and deuteranopia (no functional M-cones). These can include deletions of the opsin genes from nonhomologous recombination, point mutations like C203R or P307L that inactivate the opsin, a nonsense mutation (R247X), an intragenic deletion of whole exon 4, or the LIAVA genotype—an inactivation from homologous recombination that leaves specific amino acids in a hybrid opsin. According to the BCM International Patient Registry, about 35% of cases stem from such two-step mutations affecting both opsin genes. The remaining 55% are caused by a deletion of the locus control region (LCR), which prevents either opsin gene from being expressed. Another retinal disorder linked to the Xq28 region is Bornholm Eye Disease, where the point m