October 23, 2016 – New research shows that people with blue eyes have a single, common ancestor. Scientists have tracked down a genetic mutation which took place 6,000-10,000 years ago and is the cause of the eye color of all blue-eyed humans alive on the planet today.
Variation in the colour of the eyes from brown to green can all be explained by the amount of melanin in the iris, but blue-eyed individuals only have a small degree of variation in the amount of melanin in their eyes. Research shows that people with blue eyes may have a single, common ancestor. A team at the University of Copenhagen has tracked down a genetic mutation which took place an estimated 6-10,000 years ago and is the cause of the eye colour of all blue-eyed humans alive on the planet today.
A mutation in the HERC2 gene adjacent to the OCA2 gene, affecting OCA2’s expression in the human iris, is found common to nearly all people with blue eyes. The OCA2 gene codes for the so-called P protein, which is involved in the production of melanin, the pigment that gives colour to our hair, eyes and skin. The switch, which is located in the gene adjacent to OCA2 and which may just be a single SNP in intron 86 of HERC2 gene, namely rs12913832, does not, however, turn off the gene entirely, but rather limits its action to reducing the production of melanin in the iris — effectively “diluting” brown eyes to blue. The switch’s effect on OCA2 is very specific therefore. If the OCA2 gene had been completely destroyed or turned off, human beings would be without melanin in their hair, eyes or skin colour — a condition known as albinism.
Variation in the colour of the eyes from brown to green can ilall be explained by the amount of melanin in the iris, but blue-eyed individuals only have a small degree of variation in the amount of melanin in their eyes. It seems that all blue-eyed individuals are linked to the same ancestor. They must have all inherited the same switch at exactly the same spot in their DNA. Brown-eyed individuals, by contrast, have considerable individual variation in the area of their DNA that controls melanin production.
Nature shuffles our genes
The mutation of brown eyes to blue represents neither a positive nor a negative mutation. It is one of several mutations such as hair colour, baldness, freckles and beauty spots, which increases nor reduces a human’s chance of survival. It is as if nature were constanly shuffling the human genome, creating a genetic cocktail of human chromosomes and trying out different changes as it does so.
An even more exciting example of nature’s gene shuffling migth be heterochromia. Heterochromia is a difference in coloration, usually of the iris but also iof hair or skin and provides for a spectacular phenotype, leaving space for all sorts of metaphysical and parapsychological speculation about the person who is a carrier. Heterochromia is a result of the relative excess or lack of melanin. It may be inherited, or caused by genetic mosaicism, chimerism, disease, or injury.
Heterochromia of the eye (heterochromia iridis or heterochromia iridum) comes in three flavours. In complete heterochromia, one iris is a different color from the other. In sectoral heterochromia, part of one iris is a different color from its remainder and finally in “central heterochromia” there are spikes of different colours radiating from the pupil.
Though multiple causes have been posited, the scientific consensus is that a lack of genetic diversity is the primary reason behind heterochromia. This is due to a mutation of the genes that determine melanin distribution at the 8-HTP pathway, which usually only become corrupted due to chromosomal homogeneity.
Hans Eiberg, Jesper Troelsen, Mette Nielsen, Annemette Mikkelsen, Jonas Mengel-From, Klaus W. Kjaer, Lars Hansen. Blue eye color in humans may be caused by a perfectly associated founder mutation in a regulatory element located within the HERC2 gene inhibiting OCA2 expression. Human Genetics, 2008; 123 (2): 177 DOI: 10.1007/s00439-007-0460-x