Each human being has different eye color - some have brown eyes, some have black, green or blue eyes. What is the mystery behind different eye colors - black, brown, blue or green? What defines unique pattern seen in our eyes? Why eye color and pattern of one person differs from another? Are our genes responsible for eye color? Is eye color inheritable from parent to offspring?
Let's explore further to find out answers to the above questions.
What defines color of our eyes?
The small black dot in the centre of eye (pupil) is surrounded by a colored region called Iris. There are specialized cells known as Melanocytes which are responsible for the color of iris. These melanocytes produce a pigment called MELANIN which is not only responsible for our eye color but also for our skin and hair pigmentation. This melanin pigment is stored in intracellular compartments called as Melanosomes. The total amount of melanin present in melanosomes defines the color of our eye, hair or skin.
Role of Genetics in Eye Color
Are there any specific genes in the human genome which determine color of eye? Can eye color in humans be passed on from parent to offspring as a result of inheritance?
Eye color is determined by variation in genes which are responsible for production, transport and storage of pigment Melanin. The total amount of melanin stored in the cells (Melanosomes) define the color of our eye. Higher the amount of pigment Melanin present in the cells Melanosomes, darker is the color of the eye.
Example: People with brown eyes have more amount of pigment Melanin present in their iris in comparison to people having Blue eyes who have lesser quantities of Melanin present in the iris.
- OCA2 and HERC2 genes
Out of 23 pair of chromosomes present in human genome, Chromosome 15 plays a vital role in determining eye color.
Chromosome 15 has a particular region having two specific genes - OCA2 and HERC2, located close to one another. Variations in these two genes - OCA2 and HERC2, are responsible for different color of our eye.
Gene OCA2 produces a special protein called P-Protein which is involved in the maturation of melanosomes, the cells which produce and store melanin. Hence, Protein-P and in turn Gene OCA2 plays a crucial role in controlling the amount and quality of melanin produced and stored in Iris, thereby determining the eye color. Certain variations or polymorphisms in OCA2 gene leads to decrease in the production of P-protein hence reducing the amount of melanin present in the iris. Thus, any variation in OCA2 leads to lighter-colored blue eyes instead of dark-colored brown eyes.
HERC2 is another gene located close to OCA2 in the same region of Chromosome 15. HERC2 gene also known as intron 86 contain a DNA sequence which controls the expression of OCA2 gene by turning the gene on or off. The turning on or off OCA2 gene control the amount of P-protein been produced. When OCA2 gene is turned on, more amount of P-protein is produced, leading to more amount of melanin present in iris and hence to dark-colored eyes (brown eyes). When OCA2 gene is turned off, lesser amount of P-protein will be produced, leading to lesser amount of melanin present in iris and hence to lighter-colored eyes (blue eyes).
Earlier researchers were in a notion that a single gene determines the eye color. But now it is a known fact that several other genes also play small roles in determining the eye color such as ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, and TYRP1. Hence, the effect of all these multiple genes along with OCA2 and HERC2 play a key role in determining eye color in humans.
- If both parents have blue eye, can child have brown eyes?
Generally when eye color trait is transmitted from parent to offspring, the brown color of eye is dominant over the blue color.
Earlier it was thought if both parents have blue eyes, the child can only have blue eyes and could not have brown eyes but further studies have revealed that in certain cases the blue eyed parents have given birth to brown eye child. This was possible due to variation or polymorphism in genes which are involved in determining eye color.
Eye color trait is a polygenic trait and inheritance of this trait is a complex process where multiple genes are involved in determining eye color. Out of these multiple genes involved in the process, even a slight variation in a single gene could lead to a different eye color and pattern.
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