Chromosomes Involved in Color Blindness

Human beings have 23 pairs of chromosomes. Out of these 23 pairs 22 are autosomal chromosomes which are equal in both sexes and encode body functions. Only one pair consists of two sex-chromosomes which are different for men and women. The 22 pairs of equal chromosomes are numbered from 1 through to 22. The sex-chromosomes are labeled with X and Y, whereas women carry the combination XX and men the combination XY. This all sums up in a total of 46 chromosomes which make the human genome.

Karyotype

Human Karyotype

Color blindness was actually the trigger to start mapping the human genome. It all began in 1911, when red-green color blindness was assigned to the X chromosome. This was based upon the observation that color blindness is passed from mothers to their sons. Thereby the women are usually not affected because of the normal copy, the second X chromosome. Men in contrary can not oversteer the defective chromosome, because they are carrying just one X chromosome.

The project to decipher the whole human genome is these days much more advanced. Scientists are working eagerly to encode the whole approximately 30’000 genes in the human genome.

If we have a closer look at the chromosomes which are involved into color blindness, we should distinguish between the different types of color blindness because they are encoded at different places in the genome.

  • Red-green color blindness
    This term combines four different types of color blindness. Protanomaly and protanopia are caused by defective or even missing L-cones (long-wavelengths). In opposite defective or missing M-cones (medium-wavelengths) are the source of deuteranomaly or deuteranopia. The genes encoding the L- and M-cone photopigments are located side by side on the X chromosome. Because of the genes are highly homologous and adjacent to one another, recombinations between them is common and can lead to anomalous pigments.
  • Blue cone monochromacy
    As this type of monochromacy is caused by a complete absence of L- and M-cones, blue cone monochromacy is encoded at the same place as red-green color blindness on the X chromosome.
  • Blue-yellow color blindness
    Tritanomaly and tritanopia which are commonly referred to as blue-yellow color blindness are caused by defective or missing S-cones (short-wavelength). These photopigments are encoded in genes which reside on chromosome 7, an autosomal chromosome. This is why blue-yellow color blindness occures at the same rate on both sexes.
  • Rod monochromacy
    The total loss of color vision is called rod monochromacy or complete achromatopsia. In this case the retina does not have any cone cells at all. It is known to be an autosomal recessive disease and can be provoked by different circumstances. Recent studies show that it can be encoded on chromosome 2 as well as on chromosome 8. Earlier studies assigned chromosome 14 to rod monochromacy but this could not be reconstructed.

The genes encoding L-, M- and S-cone photopigments are very well understood and determined whereas the source of rod monochromacy is a topic which still needs further research. Supposably different circumstances can cause rod monochromacy.

Type Chromosome
Deuteranomaly X Chromosome
Deuteranopia X Chromosome
Protanomaly X Chromosome
Protanopia X Chromosome
Tritanomaly Chromosome 7
Tritanopia Chromosome 7
Blue Cone Monochromacy X Chromosome
Rod Monochromacy Chromosome 2/8

The table on the left hand side shows on a glance the different types of color blindness and their related chromosomes. We have at least 4 different chromosomes out of the 23 pairs which can be the source of color vision deficiencies. Further studies of the human genome will show which chromosomes carry the encoding genes for rod monochromacy as this is still a subject under research and therefore this table will maybe undergo some adjustements in the near future.

Further readings:
Genetics Home Reference: Chromosomes
National Center for Biotechnology Information Map Viewer
Homozygosity mapping of achromatopsia to chromosome 2
A locus for autosomal achromatopsia on human chromosome 8

Related articles:
Tritanopia – Blue-Yellow Color Blindness
The Biology behind Red-Green Color Blindness
Daltonism – Named after John Dalton

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