Blue cone monochromacy · S-cone monochromacy

Blue cone monochromacy

With neither L nor M cones working, vision relies on S cones and rods alone. Color discrimination is nearly absent and visual acuity is reduced.

Working cells

S cones + rods

Inheritance

X-linked recessive

Prevalence

~1 in 100,000

Associated symptoms

Low vision, glare, myopia, nystagmus

How it works

When the region that controls the L/M photopigment gene array on the X chromosome is lost, or both genes are disabled, the L and M cones can't function. All that remains are the sparse S cones and the rods.

Because it is X-linked recessive, it occurs almost exclusively in men.

S cones are nearly absent from the fovea, so, much as in achromatopsia, there is reduced acuity, glare sensitivity, and nystagmus, and myopia is common.

What you see

In bright light, vision is almost entirely light and dark, but in dim, twilight conditions some blue-yellow discrimination may remain thanks to differences between the rod and S-cone signals.

The table below is an approximation that keeps only luminance. Actual vision may differ from it depending on lighting and the individual.

Simulation: an approximation that removes color and keeps only luminance (Rec.709). Actual vision may vary with lighting and the individual.

How common is it?

It is estimated to affect fewer than about 1 in 100,000 people (MedlinePlus Genetics).

Its symptoms resemble achromatopsia and the two are sometimes confused, but the inheritance pattern (X-linked) and causal genes differ. Family history is important for diagnosis.

Easily confused colors

• Most colors of equal brightness
• Red/orange ↔ dark gray

Everyday tips

• Tinted lenses that reduce glare and magnification aids help.
• If men in your family have similar symptoms, genetic counseling can help with diagnosis and family planning.
• Letting your school or workplace know you need visual support can lead to accommodations in how materials are provided.

Take the color vision check → Open in simulator →