Eyes are the sensory organs of vision. The very first thing that we notice about someone’s eyes is the color of the eyes. Eyes attain a color when the iris gets pigmented by melanin. Melanin is synthesized by melanocytes and stored in melanosomes.
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The eye color is majorly categorized as brown, blue, green, hazel, and gray. Different shades of eye color are attained by the blending of white collagen fibers with the amount of melanin present in the front layer of the iris.
The quantity of melanin in the first layer of the iris determines the eye color.
Dark eye color: This is due to the higher levels of melanin pigment in the iris.
Light eye color: Many eyes are colored with lighter shades; this is because the stroma, a layer of iris performs an effect called scattering of light.
Genes Generating Eye Color
Each person gains 46 specific chromosomes from the parent, which outputs 23 pairs. The chromosomes are comprised of genes which contain DNA. Chromosome 15 contains many numbers of genes among which two major genes are responsible for the production of melanin.
OCA2 gene: It is a melanosomal transmembrane protein, also called the P-gene. The main function of the OCA2 gene is the production of a specific protein called P-protein, which majorly involves in melanin synthesis. Any mutation in the OCA2 gene results in the blockage of P-protein production, which causes a distinctive trait of albinism. OCA2 allele (a specific modification in the gene) regulates the generated OCA2 RNA, thereby triggering the levels of P-protein.
Individuals with brown eyes have alleles that cause increased levels of P-protein, which means high melanin content in the iris. Persons with blue eyes have the allele that causes a fall in their P-protein concentration, leading to low melanin pigment in the iris.
HERC2 gene: HERC2 is a protein-coding gene found close to the OCA2 gene. It is also called intron 86. The action of OCA2 is regulated by the HERC2 by making it active or inactive as required. A single nucleotide polymorphism (SNP) in HERC2 decides the color of the iris.
Blue eyes are due to the replacement of tyrosine T with cysteine C (CC allele) in the specified region of intron 86, thereby suppressing the OCA2 transcription, which in turn causes low melanin content in the iris. The replacement of cysteine C with tyrosine T (TT allele) in the certain area of intron 86 results in brown eyes.
Scientists have concluded that TYRP1, ASIP, ALC42A5, SLC24A4, SLC24A5, SLC245A2, TPCN2, and TYR are the eight different genes along with OCA2 and HERC2, which are responsible for the continuum of eye color. These genes take part in the melanin pathway where they increase or decrease the melanin levels to yield brown or blue eyes, respectively.
Inheritance of Human Eye Color
Scientists have constructed a model using two genes—EYCL3 and EYCL1 (together makes a pair) by which the inheritance of eye color (brown, blue, and green) can be explained.
- EYCL3: Also called as bey2 gene, is located on chromosome 15. This gene has brown and blue alleles.
- EYCL1: It is otherwise known as a gey gene. It has green and blue alleles.
- Brown eyes: Since brown is a dark color, it is always dominant over blue, even if a person is heterozygous (has one blue and one brown allele or one brown and one green allele). Therefore, the person gets brown color in eyes. About 55% of people in the world have brown eyes.
- Green eyes: In accordance with the gey gene, green is more dominant compared to blue. When there is one green allele on chromosome 19 and the other allele is blue, it results in the green eye color. People with green eyes are mostly seen in northern and central Europe.
- Blue eyes: The eyes attain a blue color only when all the four alleles (2 OCA2 and 2 HERC2) are blue. Blue eyes are most common in Sweden.
Thus, the above model shows that brown is dominant over green and blue, green is dominant over blue and finally, blue attains a recessive trait.
Curious Eye Colors
- Red or violet eye color: Individuals with red or violet eye color suffer from a condition called albinism. In this condition, the capability of synthesizing and storing the melanin content is completely lost. Genetically, the mutation of TYR, OCA2, TYRP1, and SLC45A2 genes are responsible for the peculiar features of albinism.
- Mixed colors: The condition in which a person has one eye color partially or completely different from the other is called heterochromia. This happens when the color of one eye changes from blue to brown or brown to green. This results from unequal levels of melanin caused by the mutation of some cells in the iris. Trauma to the eye during birth also becomes a rare cause of heterochromia.
Researches and studies are still ongoing regarding the genes responsible for the hazel and gray color of eyes.