Eye Color Genetics Chart: Fun Facts & Unpredictable Combinations

Expectant mothers have so many questions about the future characteristics of their kids. After all, approximately nine months is a really long time to wait to find out the gender of the baby as well as all the other characteristics. Wouldn’t it be nice to know ahead of time so that one can plan ahead? In fact, technology enables expectant mothers to know the gender of their baby as early as 10 weeks into the pregnancy. There is no reason not to know their baby’s other characteristics, such as eye color, for example.

The human eye color depends on the amount of pigment (melanin) that is present and how it is distributed. The more of this pigment someone has in their iris (the diaphragm of the eye that determines the amount of light that comes in), the darker their eyes will be. Light blue, for example, contains much less iris pigment than dark brown eyes, and there are numerous hues of eye color between those two extremes.

Predicting the Eye Color of a Newborn

Historically, eye color charts were once used to “predict” the eye color of future children of a couple. The most basic charts differentiated between brown eyes (dominant), green eyes (less dominant), and blue eyes (least dominant). Though this is basic, it is fairly accurate and simple to follow to determine the eye color of the population. However, the genetics and subtle variation behind eye color turned out to be a lot more complicated than initially thought, which is why eye color charts are not as appropriate a predictive measure of a child’s eye color as once thought. This is because it is no longer possible to simply calculate the odds of the baby’s eye color based on its parents and grandparents. An example that supports this is that there are families in which several generations have had brown eyes whose offspring end up having green or blue eyes.

It’s all in the Genes

It’s all in the Genes

So, what are the genetics behind eye color and can it be determined? First of all, however, let us briefly discuss genes (inherited trait). We receive our genetic material in pairs, in which one half comes from the mother and the other comes from the father. The interesting part is that each gene in the pair may differ slightly. Furthermore, one gene may override the other gene, meaning that it is dominant. However, a recessive gene only manifests itself as a trait when both genes in the pair are recessive. Eye color is an example of this scenario, whereby blue color is a trait that is encoded by a recessive gene and brown is a characteristic—or phenotype—that results from the expression of a dominant gene.

Typically, eye color in humans is the result of one gene that comes in two versions or alleles, namely brown (B) and blue (b). The alleles of the other gene are green (G) and blue (b). Those who are heterozygotes (mixed alleles), Bb and Gb will always have brown and green colored eyes, respectively, as brown and green are dominant alleles. In order for blue colored eyes (the recessive allele)to become prominent, the child would have to be homozygous (not mixed) and have two bb alleles. This is because the allele for blue eyes is recessive, which is why two alleles are needed to culminate in a trait. While this model has so far been used to explain much of how eye color is inherited, it is not enough to address the scenario of how blue-eyed parents can have a brown-eyed child.

This is most easily explained by the fact that there are two distinct genes that control the eye color trait. One is OCA2 and the other is HERC2. The former controls the production of a protein called the P protein that is found in melanocytes or cells that produce the pigment melanin. This pigment is responsible for giving eyes, hair, and skin its color. The HERC2 gene encodes a large protein that is involved in the color variation in skin, hair, and eyes by virtue of pigmentation variability. The two genes need each other to work, which is at the heart of why they can be used to explain how blue-eyed parents can have a brown-eyed child. Specifically, pigment production depends on the function of both genes, whereby a functioning HERC2 is required to turn on OCA2 which will then end up in the production of pigment. In other words, the two genes need each other to make pigment.

Furthermore, it is clear how an individual with a lack of HERC2 function will have blue eyes (no pigment is made because OCA2 is not turned on). This is also true the other way around—when OCA2 is not functioning, an individual will have blue eyes irrespective of the function of HERC2.

By virtue of the interdependence of the two genes, some individuals may be carriers of the dominant trait such as brown eyes. If these carries have blue eyes, combining their genetic material could actually lead to children with brown eyes. Amazing, isn’t it!

To make things a bit more complex, both of the genes are located on one and the same chromosome and are therefore close to one another. However, discussing how their close proximity on chromosome 15 may affect eye color is largely beyond the scope of this blog post.

Final Thoughts

Some interesting eye color facts: the majority of Caucasian babies of the world’s population are born with blue eyes that will change color within the baby’s first year of life. The term “green-eyed monster” was used by Shakespeare to refer to the feeling of jealousy. Today, green is one of the more desirable eye colors. Depending on the pupil size and the lighting, hazel eyes can appear to change in color depending on the circumstances. While brown is the most common eye color, lighter variations such as hazel and light hazel are very rare and strikingly beautiful.

Laura Day
 

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