Chromosomes Involved in Color Blindness
- Posted by Daniel Flück on June 2nd, 2006 filed in Academic
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.
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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June 9th, 2006 at 18:32
It seems perfectly reasonable that Rod Monochromacy could be caused by more than one “knockout mutation”, so maybe both 2 *and* 8 bear relevant genes, perhaps common to cone development in general.
June 9th, 2006 at 21:53
What amazes me a bit, that the genes encoding S-cones are well determined and understood. In contrary rod monochromacy is not understood that well concerning genes which could be a cause. Of course it could be both chromosomes involved, maybe even more. It still needs more research to find out about it in detail.
November 11th, 2006 at 0:59
is there a difference in the karyotype of a color blind individual and that of a normal individual??
November 11th, 2006 at 21:29
As far as I understand it, there is no difference in the karyotype of a colorblind person and one with normal vision (aren’t colorblind people normal? :-)
The definition of karyotype is: A photographic representation of the chromosomes of a single cell, cut and arranged in pairs based on their banding pattern and size according to a standard classification.
Color blindness is just a slight mutation in some genes of some chromosomes. This mutations can’t be seen in a big scale picture of the whole chromosomes like the karyotype is.
May 8th, 2007 at 18:46
[…] According to a report on the genetics of color blindness from the Howard Hughes Medical Institute, around 7% of the male population can’t tell the difference between red and green or see those two colors differently from the rest of us. Of course, philosophers would say that there’s no way we can tell whether my perception of colors is really the same as yours but that’s not really for geneticists to worry about. Color blindness is more common in males because the mutant genes for red and green color receptors are on the X chromosome. Daniel, of course, has more on the chromosomes involved in color blindness. […]
May 13th, 2008 at 1:35
wow thx to that infomation i no ima get a A+ on ma project!!lol