Expert: Dana Krempels, Ph.D. - 10/24/2008

Question
My sister-in-law is pregnant and going to have a boy.  Her father is completely colorblind.  I don't know the technical term for this but I know he sees absolutely no color whatsoever.  My question is, what is the liklihood that her son will be born colorblind?  Is there a percentage or ratio you could give me?  She does not know if she is a carrier or not.  Thanks,  Jennifer

Answer
Dear Jennifer,

Your sister-in-law's father suffers from a very rare condition known as monochromacy, or complete color blindness.  Unlike red/green color blindness, it is not an X-linked inherited condition, and its heritability depends on the exact *type* of monochromatism.  

You can read more about the condition here:

http://www.colblindor.com/2007/07/20/monochromacy-complete-color-blindness/

and here, from our old pal, Wikipedia:

Monochromacy is the condition of possessing only a single channel for conveying information about color.[13] Monochromats possess a complete inability to distinguish any colors and perceive only variations in brightness.[13] It occurs in two primary forms:

  1. Rod monochromacy, frequently called achromatopsia, where the retina contains no cone cells, so that in addition to the absence of color discrimination, vision in lights of normal intensity is difficult. While normally rare, achromatopsia is very common on the island of Pingelap, a part of the Pohnpei state, Federated States of Micronesia, where it is called maskun: about 1/12 of the population there has it. The island was devastated by a storm in the 18th century, and one of the few male survivors carried a gene for achromatopsia; the population is now several thousand, of whom about 30% carry this gene.
  2. Cone monochromacy is the condition of having both rods and cones, but only a single kind of cone. A cone monochromat can have good pattern vision at normal daylight levels, but will not be able to distinguish hues. Blue cone monochromacy (X chromosome) is caused by a complete absence of L- and M-cones. It is encoded at the same place as red-green color blindness on the X chromosome. Peak spectral sensitivities are in the blue region of the visible spectrum (near 440 nm). They generally show nystagmus ("jiggling eyes"), photophobia (light sensitivity), reduced visual acuity, and myopia (nearsightedness).[14] Visual acuity usually falls to the 20/50 to 20/400 range.

Cone monochromacy is far more common in the general population, and as you can see above, it is--like red/green color blindness--carried on the X chromosome.

If your sister-in-law's father has blue-cone monochromacy, then there is a 50% chance that his daughter inherited it and will pass it on to her child.  She does not express it because her other X chromosome has a normal version of the gene, which masks the expression of the mutant form her father has.

If she *is* a carrier of this mutant allele of the color vision gene, then her daughter will be a carrier, like herself.  If she has a son, the child will express the mutant condition, as his grandfather does.

But there's only a 50% chance that your sister-in-law is a carrier--and we don't know for sure what type of complete color blindness the grandfather has.  Unless we know a bit more, it's difficult to predict the outcome in the child.

Hope this helps for starters.

Dana