Hello, my name is Evan Becquet, and I am an aspiring optometrist at Mount Aloysius College here in Cresson, PA. Since high school I have been infatuated with eyes and eye color, I mean it is after all one of the first things you notice when meeting someone new. Eye color is something that is unique to each human being and it really grew my interest on how genetics play a part in the transfer of eye color from parent to offspring.
It is important to first understand the process of how eyes receive their color in the first place. Currently it is thought that there are approximately 6 genes that determine eye color, meaning that there can be a lot of discrepancy when it comes to eye color. This is done by influencing the production and transport of the pigment melanin. To put it in perspective, brown eyes contain a large amount of melanin in the iris whereas blue eyes contain little to trace amounts. Typically, newborns are born with blue eyes because melanin has not fully developed in the eyes yet meaning eye color can take anywhere from 6 months to 3 years to fully develop! Green eyes however are a different story. The eye color green is actually considered a mutation in which the eye produces more melanin than in blue, but not as much as in brown. This causes light to scatter in the iris and make the eyes appear green.
While on the topic of ocular mutations, I would like to talk about a non-harmful condition known as a heterochromia. The reason I mention this is because it is something that I am personally affected by. Heterochromia is known as the phenomenon of having two different colors of the iris and there are three different types: Complete, Sectoral, and Central.
Complete heterochromia is when an individual has 2 completely different colored irises in the right and left eyes. This is extremely uncommon and there are thought to only be roughly 200,000 people across the United States that have this condition. Sectoral heterochromia is when one section of the iris is different color than the rest. It is believed that only one percent of the entire world population are affected by this. the final condition, Central heterochromia the least rare of the conditions, is when the center of the iris is a different color than the rest of the eye. In my case, I have blue eyes with brown rings around my pupils as seen in the picture below.
Naturally, parental eye color plays a crucial role in the eye color of the offspring but it is much more complicated than just a simple Punnett square. To put it in perspective using the Punnett square method, say a parent is heterozygous (contains both alleles) for brown eyes and has a green eyed spouse that is also heterozygous. They could quite possibly produce blue eyed offspring if the recessive alleles are expressed. This is only an example of the complexity of eye color determination and doesn't account for things such as genes that can modify melanin production and even suppress dominant genes. As previously stated, brown is the dominant eye allele so naturally it is the most prevalent eye color in the world. It is thought that somewhere between 55-79 percent of the whole population have brown eyes. Blue eyes however is the recessive trait and can only account for 8 percent of the population which seems minimal until you take into consideration that green only account for 2 percent!
Eyes are something unique to each person and the genetic variation of eye colors throughout generations is fascinating. Future research may provide some very interesting genetic insights as to the exact process of eye color determination and how traits are passed as well as lead to potential methods of more accurate eye color prediction.
Sources
https://askabiologist.asu.edu/questions/eye-color-genetics
Eye picture: Taken March 3, 2022 by Evan Becquet
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