Story behind AMBER EYES??
Amber eyes, these are very unique and special eyes because their iris are in the shade of brown mixed with gold and copper strings, which look very beautiful and unique when seen in sunlight. I decided to write about this because it is one of the rare eye colours and only approximately 5% of the world's population have amber eyes, which are rare. There are other rare eye colours as well which I would like to share with you all on my upcoming blog posts. But for today I would first like to start with AMBER EYES.
WHAT ARE EYE COLOURS, AND HOW DOES THE GENES ARE INVOLVED IN THE CHANGE IN THEIR COLOURS?
Eye colour is a polygenic phenotypic character. The eye colour of a person is determined either by the pigment of the iris or by the scattering of the light by the turbid medium in the stroma of the iris. Eye colour is mostly depended on the concentration of the melanin. As we all know that coloured part of our eyes is called IRIS, and the brown part of the eye is due to the presence of melanin.
"Scientists think that long ago when everyone lived in hot climates where it was sunny all round the year, back then almost everyone had brown eyes. It was believed that dark irises may have protected their eyes from being damaged by ultraviolet(UV) radiation and bright sunlight."
For amber eyes, the iris contains mostly the pigment called "lipochrome." It is believed that the change in eye colour is influenced by more than one gene. Few genes are associated with the change in the eye colour they are as follows:-
- HERC2 - HECT and, domain containing E3 ubiquitin protein ligase2. It is located in chromosome15.
- OCA2 - OCA2 melanosomal transmembrane protein. It is located in chromosome15.
- MC1R - Melanocortin 1 receptor. It is located in chromosome16.
- TYR - Tyrosinase. It is located in chromosome11.
- SLC45A2 - Solute Carrier family 45 membranes 2. It is located in chromosome5.
Single Nucleotide Polymorphisms(SNPs) in the HERC2 gene and to a lesser extent, in the neighbouring OCA2 gene were independently associated with iris colour variation. But till this date, the genetics of iris colour is still not fully understood. A recent study on the genes has confirmed that HERC2 gene is a new and significant determinant of human iris colour in addition to the OCA2 gene.
The pigment melanin present in the melanosomes can occur in two forms they are:
- Eumelanin: It is a brown-black form which is responsible for the dark colour of the iris. It is photoprotective.
- Pheomelanin: It is a red-yellow form of melanin. Gene mutations that lead to a loss in the function are associated with increased pheomelanin production, which leads to lighter skin and hair colour. Pheomelanin may contribute to UV induced skin damage by generating free radicals upon UV radiation.
Oculocutaneous Albinism2 (OCA2) is a group of rare inherited disorders characterized by a reduction or complete lack of melanin pigment in the skin, hair, and eyes. According to a report provided by the HUGO gene nomenclature committee, OCA2 encodes the human homolog of the mouse p (pink-eyed dilution) gene. This encoded protein is believed to be an integral membrane protein involved in small molecule transport, specifically tyrosine, as it is the precursor for melanin synthesis.
Melanocortin 1 Receptor(MC1R) is also known as Melanocyte Stimulating Hormone(MSH). This is an encoded protein, the transmembrane G protein-coupled receptor controls melanogenesis. When MSH binds with the receptor it activates the receptor and stimulates eumelanin synthesis. This receptor is a major determining factor for sun sensitivity and is a genetic risk factor for melanoma and non-melanoma skin cancer. There are over 30 variant alleles that are identified to be correlating with the skin, hair colour providing evidence that this gene is an important component in determining normal human pigment variation. [ This is according to the report of HGNC from NCBI on MC1R.]
Tyrosinase(TYR) is an enzyme and has a protein-coding gene type which has both Tyrosine Hydrolase and DOPA oxidase catalytic activities, and requires copper for function. Mutation in this gene results in Oculocutaneous Albinism and non-pathologic polymorphisms results in skin pigmentation variation. The human genome contains a pseudogene similar to the 3' half of the TYR gene. [This is according to a report on TYR tyrosinase by HGNC from NCBI.]
Solute Carrier Family 45 Member 2(SLC45A2) This encodes the transporter protein that mediates melanin synthesis. This transporter protein expressed in a high percentage of melanoma cell lines. Mutations in this gene are a cause of Oculocutaneous Albinism type 4, and polymorphisms in this gene are associated with variations in the skin and hair colour. Multiple transcript variants encoding different isoforms have been found for this gene. [This is acc. to the report of HGNC on SLC45A2 from NCBI.]
HOW IS THE COLOUR OF EYE REFLECTED?
- For Brown eyes: The brown colour of the eyes are reflected because eumelanin present in the front layer of an iris of the eyes. This eye colour is most commonly found, about 90% of the population has this eye colour.
- For Black eyes: The eye colour is reflected black because the eumelanin is present in much higher concentration in the front layer of an iris. Thus, the eye colour is black.
- For Blue eyes: It is found that in blue eyes the presence of melanin is much lesser than the other eye colours. and this blue colour of an eye is reflected due to the scattering of the light in the scattering layer.
- For Green eyes: This colour is present in only 5% of the population in the world. this colour is reflected by the combination of the scattered light from the scattering layer of iris with the pheomelanin that is found in the front layer of an iris, and it is also present in high concentration.
- For Red/Pink eyes: This colour of eyes are found in patients with albinism as there is no pigment present in iris of an eye. Thus, the red colour comes from the reflection of the blood vessels that are present behind the iris.
- For Amber eyes: This colour amber is very rare as we know, it is reflected as there is a medium concentration of pheomelanin present in the front layer of an iris. But this has not been confirmed by the scientists yet. As more researcher on this topic is yet needed to be done.
- Nicole Richie
- Gabriel Valenzuela
- Alexandra Liao
- Emily Harrison
- Amanda Righetti
"The bottom line is that even if you have the same colour of eyes as everyone else or different, always remember one thing that you are as unique as other, and beautiful just the way you are."
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