Eye Color : What color will my baby’s eyes be?

Introduction


The color of our eyes is a product of the pigmentation of the iris, the structure around the pupil.

The main pigment of the iris is melanin, which is a complex pigment, present in the skin, eyes and hair, whose main function is to absorb the ultraviolet rays from the sun, protecting the cells of the tissues from damage by this radiation.

Eye color is determined mainly by the amount of melanin in the iris, but also by the way light interacts with other pigments and substances in the eyes, such as collagen, blood vessels, and lipochrome. In general, people with more melanin have darker eyes; people with less melanin have lighter eyes.

There is no blue color pigment in eyes. Some people have blue eyes in the same way that the sea and sky are blue: the structures in the iris absorb the longer wavelengths of light and reflect the shorter wavelengths, which are blue.

The amount of melanin in the iris is genetically determined, with brown being the most common color, present in almost 80% of the world’s population. Other possible eye colors are gray, green, blue, amber, and hazel.

Eye Color Defining Genes

Since eye color is genetically determined, until a while ago we thought it was more or less simple to estimate what color eyes a baby would have based only on the eye color of his parents and grandparents.

We learned in school that brown eyes are governed by dominant genes, and blue or green eyes are governed by recessive genes. But the single gene model for eye color is not the correct one.

The classic genetic model that explains and predicts the color of an individual’s eyes is based on two genes. They are:

  • OCA2 – gene with dominant trait for brown eyes (B) and recessive for blue eyes (b).
  • EYCL1 or gey – gene with dominant trait for green eyes (G) and recessive for blue eyes (b).

* (B) brown, (G) green e (b) blue.

In this model, allele (B) is always dominant, allele (G) is dominant over (b), and allele (b) is always recessive.

Therefore, the color of a person’s eyes varies according to the possible combinations between these two genes. The table below shows the variations that exist.

Genes (OCA2 and gey)eye color
bb and bbBrown
BB and GBBrown
BB and GGBrown
baby and babyBrown
Bb and GbBrown
Bb and GGBrown
bb and ggGreen
bb and bbGreen
bb and bbBlue

Conclusions:

  • If you have one (B) allele in the OCA2 gene, your eyes will be brown, no matter what other alleles you have.
  • If you don’t have the (B) allele in the OCA2 gene and you have one (G) allele in the gey gene, your eyes will be greenish.
  • If you only have the (b) allele in both genes, your eyes will be blue.
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This model explains why brown eyes are much more common than green and blue eyes.

Can brown-eyed parents have blue-eyed children?

Yes, it is perfectly possible, and relatively common, for brown-eyed parents to have blue-eyed children.

As the alleles for brown eyes are dominant, it is common for someone with dark eyes to carry the genes for light eyes. If that person has a child with someone else who also carries the genes for blue or green eyes, there is a chance that the child will only receive the recessive genes and be born with blue or green eyes.

Examples:

  • Father “Bb bb” + Mother “Bb bb”: the child can be born “BB bb” (brown eyes), “Bb bb” (brown eyes carrying the gene for blue eyes) or “bb bb” (blue eyes).
  • Father “Bb bb” + Mother “Bb GG”: the child can be born “BB Gb” (brown eyes), “Bb Gb” (brown eyes carrying the gene for green eyes), “bb Gb” (green eyes) or “ bb GG” (green eyes).

Can blue-eyed parents have brown-eyed children?

Almost everyone believes that blue-eyed parents will necessarily have blue-eyed children. This is true in 99% of cases, but as with everything else in genetics, there are always exceptions. Just under 1% of parents with blue eyes have at least one child with brown eyes.

According to the genetic model described above, blue-eyed mother and father would always have blue-eyed children. After all, if the father is “bb bb” and the mother is “bb bb”, the son could only also be “bb bb”. The problem is that the model described above is not complete. It works well for almost all cases, but not for all.

It turns out that the OCA2 gene is actually two genes, called OCA2 and HERC2, which are located on chromosome 15. The reason these two genes can explain brown-eyed children with light-eyed parents is that the two they need each other to function. If one of the genes is inactivated or defective, there will be no production of melanin in the iris.

Some blue-eyed people have genes for brown eyes, but at least one of them is “broken” and doesn’t work as it should. And it makes no difference having the gene if it doesn’t work. It’s like not having it.

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The illustration below helps you understand how a person with the gene for brown eyes can have blue or green eyes.

Therefore, if the father and mother are blue-eyed people by default in one of the genes for brown eye, and their child receives only the genes without a defect, he may be born with brown eyes. This situation is obviously rare, which is why it only occurs in less than 1% of cases.

Another possibility is that a parent with blue eyes has the perfect gene for brown eyes, but is turned off by some other gene. In that case, the perfect gene for brown eyes can be passed on to the child without the gene that inactivates it. Therefore, as the gene for brown eyes is dominant, the child will have brown eyes.

What color will my child’s eyes be?

It’s impossible to predict with certainty what your child’s eye color will be, but we can estimate the odds. The table below shows the percentages according to the parents’ eye color.

Why are there other eye colors besides blue, green and brown?

Because the genetic model explained above remains incomplete. It is not just two or three genes that define the production of melanin in the iris. These are the main ones, but there are several others that exert a weaker effect synergistically or antagonistically to the OCA2 and HERC2 genes.

Examples of genes that also influence eye color include ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR and TYRP1. The effects of these genes combined with those of OCA2 and HERC2 is that they produce a continuum of colors, generating eyes of gray, violet, blue, green, honey, hazel, light brown, or dark brown.

Why are babies not born with the definitive eye color?


Light-skinned babies are usually born with grayish or blue eyes because melanin production is still very small at birth.

Only from 3 to 6 months of age will the baby’s eye already possess enough melanin to have a color similar to the color it will have throughout its life.

Curiosities


Gray eyes


People with gray eyes are those with very little melanin in the iris. They make up about 3% of the population and are most common in northern and eastern Europe.

Actually, gray eyes are very light blue eyes. Depending on the color of the clothes, the environment, and the intensity of light, the grayish color can become more bluish or greenish.

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Green Eyes

Truly green eyes are rare, and are present in only 2% of the population.

What we usually call green eyes are actually blue eyes with a greenish tint or light brown eyes with a greenish tint.

True green eyes arise by the scattering of light through the interaction between the blue color of low melanin and the yellowish color of a pigment called lipochrome. Green eyes can have more yellowish and more bluish regions, depending on how the light rays fall on them.

This type of eye is most common in northern Europe and in red-haired people.

Red eyes

People who suffer from albinism have so little melanin in the iris that it is almost transparent. In these individuals, the eyes may be reddened due to staining of the retina and the blood vessels that pass behind the iris.

Honey-colored eyes

Honey or amber eyes occur in 5% of the world’s population. This eye coloration is also caused by the presence of the lipochrome pigment, which is yellowish. People with little melanin and a lot of lipochrome usually have this coloration.

Although rare in humans, honey-colored eyes are quite common in the animal kingdom, especially in dogs, felines, fish and birds.

People with one eye of each color

A genetic anomaly called heterochromia can cause each eye to have a different color as a result of different concentrations of melanin in each iris.

Heterochromia can be complete, when each eye is entirely one color, or partial, when one eye is only partially another color.

Heterochromia is common in some animal species, such as dogs and cats, but it is rare in humans.

Surgery to change eye color

A few years ago, news emerged on social media of a colored silicone implant surgery in the iris that would be able to permanently change the color of the eyes.

This type of surgery is not approved by any ophthalmology association in the world and is known to be unsafe.

Studies carried out with patients undergoing this type of surgery have shown that serious complications can arise, including:

  • Loss of sight.
  • Glaucoma.
  • Cataracts.
  • Corneal damage, which can be severe enough to require a corneal transplant.
  • Inflammation of the iris or surrounding areas, causing pain, blurred vision, and tearing.

References