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Eye color is mainly determined by the type and amount of pigment in the irises called melanin. The genetic makeup of someone’s parents also heavily contributes to eye color.
The genetic variations that control the production, distribution, or concentration of melanin play a role in determining an individual’s eye color.
Whether a person has blue eyes or brown eyes is more than just the simple Mendelian inheritance model.
Mendel's Laws explain how people inherit traits from their parents.
The first law says that parents give one copy of a trait to their children. These copies split apart when making new generations.
The second law says that different traits are inherited independently. You can get a mix of traits from your parents.
These laws help scientists understand how traits like eye color are passed down from one generation to the next.
While Mendel's laws suggested a simple dominant-recessive relationship between brown eyes and blue eyes, modern research has found a complex relationship. It found the interaction between multiple genes (polygenic) and factors contribute to the diversity of eye colors.1
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Genes determine eye color through their influence on melanin production, distribution, and concentration in the iris.
The primary pigments involved are eumelanin (brown or black) and pheomelanin (red or yellow).
The complex genetic process involved in determining eye colors involves the following factors:
Genes dictate the production of melanin, the pigment responsible for eye, skin, and hair colors. Variations in specific genes, such as Oculocutaneous Albinism II (OCA2) and HECT And RLD Domain Containing E3 Ubiquitin Protein Ligase 2 (HERC2), impact the amount of melanin produced.
Multiple genes define if a person will have blue eyes or other colors. In addition to OCA2 and HERC2, other genes like ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, and TYRP1 play roles in melanin production and distribution.2 These genes interact with each other in complex ways to create a selection of eye colors even as rare as green eyes.
Some genes control the expression of other genes. For instance, HERC2 regulates the activities of OCA2. The discrepancies in gene regulation can influence gene expression. As a result, it can alter melanin production, leading to people having blue eyes or brown eyes.
The balance between eumelanin and pheomelanin can affect eye color. Brown eyes have more eumelanin, while blue eyes and green eyes have less eumelanin and more scattering of light due to the lack of pigmentation.
While simple Mendelian inheritance patterns can help determine eye color, the complexity arises from the fact that multiple genes are involved. Eye color inheritance doesn't follow a strict dominant-recessive pattern, allowing for a wide range of possible results.
Different populations have a range of combinations of genetic variants that influence eye color. People’s eye color can be affected by their genetic heritage and the diversity of genes in their ancestral backgrounds. Coming from a specific ancestry can usually help determine whether someone will have brown eyes or not.
Lighting conditions can influence how eye color is perceived. Eye colors might appear slightly different under different lighting conditions. Blue eyes may appear lighter in a different environment.
Multiple genes contribute to producing different eye colors among humans. Variations in these genes can influence melanin production, transport, and concentration. Therefore, various genetic discrepancies can produce blue, brown, or green eyes.
Eye color is polygenic. Multiple genes involved can influence this trait, including:
Genetic mutations can influence eye color by affecting melanin production, distribution, and regulation. Changes in the DNA sequence of a gene can lead to variations in the amount and type of melanin in the iris.
Here’s how genetic mutations can lead to unique eye shades and patterns:
There are no specific genes exclusively responsible for producing blue eyes or brown eyes.3 Eye color is a polygenic trait, which means multiple genes interact and can affect its outcome.
While certain genetic variations, especially those that handle melanin processes, these genes collectively contribute to a wide range of eye colors.
The intricacy of eye color determination involves the relationship of multiple genes, their variations, and regulatory elements. Many factors are in play that contribute to the diversity of human eye colors.
Eye color inheritance can be understood through navigating the relationship among multiple genes, their variations, and complex interactions.
In the traditional sense, understanding eye color inheritance in the simple Mendelian trait says one gene dominates another. However, this has evolved into a more intricate model that accounts for the involvement of multiple genes.
Parents have an effect on their child’s eye color through a combination of genetic interactions and variations.
Two blue-eyed parents may have a child with blue eyes, but not every time. Sometimes, the kid can end up with brown eyes. Still, the mother and father contribute to their offspring's final eye color.
It is a common misconception that one parent may strongly influence their kid’s eye color. However, the genetic makeup of both affects their child's final eye color outcome.
Consequently, predicting the exact eye color of a child can be challenging due to the complex interaction of multiple genes, regulatory mechanisms, and genetic diversity from both parents. Hence, two blue-eyed parents don’t automatically have a kid with blue eyes.
Some people can have different eye colors from both parents due to the complex nature of eye color inheritance.
While the basic Mendelian inheritance model suggests a direct dominant-recessive pattern, the reality is more complicated.
Eye color is influenced by multiple genes. It’s not a simple dominant-recessive pattern like the Mendelian inheritance model.
Therefore, multiple genes can have an effect on the outcome. The final eye color can be tricky to predict with more genes involved.
Genetic mutations and variations from past generations can also result in a person’s unexpected eye color. It means parents with blue eyes can still have a brown-eyed child.
Eye color can sometimes be linked to specific genetic conditions. Some of these conditions include:
People with albinism, a genetic disorder affecting melanin production, often have light-colored eyes due to the lack of pigmentation. Moreover, they have light or white hair and skin.
In addition to albinism, other genetic diseases can affect eye color. These are:
Eye color distribution varies across different regions and populations. However, brown eyes are the most common in the world.4
Let’s take a look at the estimate below on how common is each eye color:
Eye color can change over time due to genes, trauma, disease, or medication.5
Eye color change is more likely to happen in childhood. Newborn babies usually have bluish-gray eyes due to the lack of pigmentation. As the amount of melanin in their irises increases, eye color change can happen. Later on, their eyes gradually become darker as they age.
While most people's eye color becomes stable and permanent by around the age of 6, some may experience subtle changes in their eye color throughout their lives.
Since genetics contribute to determining eye color, predicting a baby’s eye color can be tricky. While an educated guess can be made based on the parent’s eye color history, the answer isn’t absolute.
It is essential to understand basic inheritance patterns. Generally, brown eyes are dominant over blue eyes, and green eyes or hazel eyes can result from a combination of genes.
Genetic diversity can lead to unexpected variations. Therefore, a baby's eye color might follow a complex pattern.
Green is the rarest eye color.6 Only 2% of the world’s population has this color.
The unique combination of the amount of melanin in the irises and how the light scatters off the eyes produces the green eye color. The optical effect of light scattering off the melanin is a distinct way that makes a few eyes green.
Eye color on its own doesn’t impact vision. Melanin concentration in the irises dictates eye color. Typically, it is a cosmetic trait. Moreover, this pigment protects the eyes from excessive sunlight and harmful UV rays.
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