Eye color – inheritance, classification, heterochromia. What affects eye color?

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Eye color depends on the type and amount of pigment in the iris. There are many possibilities, there are even 80 variants of eye color according to some classifications. Eye color is an inherited trait. Melanin is responsible for it, i.e. the same dye that is responsible for the color of our skin or hair.

Eye color inheritance affects all ancestors, not just parents. Eye color is the result of different combinations of genes. Dominant and recessive genes play a major role. So what influences our eye color?

Eye color – what influences it?

A person’s eye color depends on how much pigment called melanin is in the front layers of the iris, that is, the structure surrounding the pupil. Specialized cells called melanocytes produce melanin, which is stored in an intracellular compartment called melanosomes. Humans have roughly the same number of melanocytes, but both the amount of melanin in melanosomes and the number of melanosomes in melanocytes are different.

Eye color depends on the amount of melanin stored in these compartments. In people with blue eyes, the minimum amount of melanin is found in a small number of melanosomes. People with green eyes have a moderate amount of melanin and a moderate number of melanosomes, while people with brown eyes have a high amount of melanin stored in many melanosomes.

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Eye color – a pattern of inheritance

Due to the number of genes responsible for eye color, the pattern of inheritance is complex. While a child’s eye color can generally be predicted by looking at the parent’s eye color, the polymorphisms that may arise mean that your baby may have unexpected eye color.

A child’s eye color depends on the pairing of genes passed on by each parent, which are believed to include at least three pairs of genes. The two main gene pairs that geneticists have focused on are EYCL1 (also called the gey gene) and EYCL3 (also called the bey2 gene).

The different gene variants are called alleles. The gey gene has one allele that gives rise to green eyes and one allele that gives rise to blue eyes. The bey2 gene has one allele for brown eyes and one allele for blue eyes. The brown eye allele is the most dominant allele and is always dominant over the other two alleles, and the green eye allele is always dominant over the blue eye allele, which is always recessive. This means that parents who have the same eye color may still develop a different eye color in their child.

For example, if two brown-eyed parents pass on a pair of blue alleles to their offspring, the child will be born with blue eyes. However, if one of the parents passes the green allele then the child will have green eyes, and if the brown allele is present then the child will have brown eyes regardless of the other three alleles.

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Eye color – classification

The most detailed classification of eye colors is the scale of prof. Michalski, which is based on the Martin scale. According to her, after taking into account the spots and areola on the iris, as many as 80 different eye colors were established.

The Martin scale is the most popular eye color classification. It assumes the following division.

Dark eyes:

1. black-brown (dark brown),
2. dark brown (dark brown),
3. brown (beer),

Transitional / mixed eyes – various pigments appear on the iris in the form of, for example, relatively evenly spaced spots or a dark rim of a light iris. This category includes the eyes:

1. light brown (light brown),
2. green-beer,
3. eyes mixed with a predominance of lighter shades:
4. light green (green-blue),
5. dark gray (more green admixture),
6. grey.

Clear eyes:

1. light gray,
2. grey blue,
3. blue,
4. light blue (cyan).
5. black-brown (dark brown),
6. dark brown (dark brown),
7. brown (beer),

Eyes mixed with a predominance of darker shades:

1. light brown (light brown),
2. green-beer,

Eyes mixed with a predominance of lighter shades:

1. light green (green-blue),
2. dark gray (more green admixture),
3. grey,
4. light brown (light brown),
5. green-beer,
6. light green (green-blue),
7. dark gray (more green admixture),
8. grey,
9. light gray,
10. grey blue,
11. blue,
12. light blue (cyan).

Eye color – brown eyes

Most people in the world have brown eyes. The brown color is the result of the high concentration of melanin in the iris, which causes more light to be absorbed and less to be reflected. For this reason, brown eyes are more naturally protected from the sun. This likely had evolutionary benefits similar to darker skin being able to withstand the sun’s rays longer. Skin color genes are closely related to eye color genes.

Although brown eyes are the most common genetic eye color, there is more genetic variation among people with brown eyes than people with blue eyes. This could explain the color differences in brown eyes. These differences come from different genes on different chromosomes that carry genetic information about eye color from our ancestors.

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Eye color – blue eyes

Originally, all people had brown eyes. About 6000 to 10000 years ago, a genetic mutation affecting one gene disabled the ability to produce enough melanin to color the eyes brown, causing blue eyes. This mutation arose in the OCA2 gene, the main gene responsible for determining eye color. Because blue eyes have survived for many generations, scientists believe they may have had some evolutionary benefitsalthough the exact cause is unknown.

Blue eyes are the result of low levels of brown melanin, not blue pigmentation. Less melanin allows more light to reflect back to wavelengths in the blue color spectrum, which in turn makes the eyes appear blue. The reason why the eyes are blue is the same reason the sky is blue. About 8 to 10 percent of people worldwide have blue eyes.

Eye color – green eyes

Only about 2 percent of the world’s population have green eyes. Green eyes are a genetic mutation that produces low levels of melanin, but more than blue eyes. As with blue eyes, there is no green pigment. Instead, due to the lack of melanin in the iris, more light is diffused, making the eyes appear green. The changes in the light make the lighter eyes appear as if they are changing colors like a chameleon.

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Eye color – hazel eyes

Hazel eyes are sometimes confused with green or brown eyes. They are not as rare as green eyes, but they are rarer than blue eyes. Only about 5 percent of the world’s population have the beer eye genetic mutation. They have the most melanin after brown eyes. The combination of less melanin (as in the case of green eyes) and a large amount of melanin (as in the case of brown eyes) makes this eye color unique.

The color combinations in shades of green, brown and gold are endless with hazel eyes, depending on the concentration of melanin. The light scatters, as is the case with blue and green eyes. As with blue and green eyes, hazel eyes can change colors depending on the light. Eye color doesn’t really change, perception does. It is unknown whether the hazel eyes developed from brown or green.

Eye color and sensitivity to light

People with blue, gray, or green eyes are usually more sensitive to light than people with brown or black eyes. In fact, people with brighter eyes often experience photophobia or sensitivity to light, causing them to squint in the sunlight.

Sensitivity to light is due to the fact that people with light eyes have less pigmentation in many layers of the eyes. As a result, they are unable to block the effects of bright light or sunlight. With this in mind, parents should keep an eye on their children when they are outside and watch when their child may need a break from sunlight. Regardless of your child’s eye color, it is best to protect their eyes when outdoors with sunglasses that provide UV protection.

Eye color – heterochromia

This is an extremely rare and unusual genetic defect that affects approximately 1% of the total population. It consists in an uneven distribution of the dye and thus a different color of the iris or irises. As a result, such a person may have one blue eye and one green eye (heterochromia iridium), or one eye may have two different colors (heterochromia iridis).

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