Research

What determines your eye colour?

Eye colours are fascinating. Some people are born with green or blue ones which appear to be highly attractive. On the other hand, for several generations, a family will have members with the same eye colour. At times, however, even within one family, one child will have a different colour than that of his/her sibling. This phenomenon is interesting to understand and has a wide explanation attached.

Medical science has tried to explain the reason for different eye colours through different theories. Though, mostly genetical there are other factors that can affect what colour a person gets at birth. Furthermore, with the advancement in research, science has allowed varying the colour as ascribed when a baby is born!

This article will answer the question relating to the determination of eye colour. It is important to recognize all the underlying explanations for this fascinating topic.

Overview

Eyes are one of the most vital organs we have. Obviously, without them, we will not be able to visualize the world around us. However, the added advantage is the fact that having them makes who we are. The eyes are the windows of the body and the eye colour increases the beauty of these windows!

Parents are often worried about what eye colour the baby will have. If the mother has a brown and the father has a hazel colour, would the baby have blue eyes? Questions like these are frequently geared towards doctors who give out all the possibilities. These possibilities are calculated through a proper mechanism which this article will extend upon later. Like with the majority of the things, science has made it easier to identify the eye colour at birth.

So your parents might not have directly chosen what colour you will have at birth. However, for certain their own eye colours had an impact on what you got!

How the eye colour is determined?

Medical scientists have identified broadly two main factors that will determine a person’s eye colour.

Iris is which is responsible for the dilation and contraction of pupil entering an eye. It performs this function by scattering the light. It contains a brown pigment called melanin. Thereby the amount of melanin that is present in the eye and the pattern of it will determine the eye colour. Therefore, a dark brown eye colour is certainly loaded with a lot of melanin.

Genes. The genetics of a person is responsible for some of the traits we have in our body. We can not do much about it because it was ascribed at birth. When the doctor tells you that a condition is a genetic disorder, it means that it was inherited from the parents. Similarly, eye colours are determined genetically depending upon the genes of the two parents. This phenomena is a never-ending process and continues for generations. However, with technological advancement, genetic engineering has become quite common. One can now change certain characteristics through certain scientific processes.

The genetic explanation for eye colour

Everything in our body is composed of millions of cells. A cell is the basic microscopic unit that a naked eye can not visually see. There is an interesting underlying biology behind the composition of cells. Each has a nucleus that consists of chromosomes, 46 of them in total. The 46 chromosomes are further divided into two pairs of 23 each. A baby that is born will inherit one single chromosome from the mother and further to come up with a pair.

To further add to the insight, every chromosome further has DNA known as genes. Genes like chromosome are present in pairs. They are what ultimately will determine a person’s eye colour. Around 15 different types of the gene will play their role in determining the characteristics. The main ones responsible for this process are OCA2 and HERC2 belonging to chromosome 15.

Alleles

The story does not end here. The alleles will finally decide what eye colour will be expressive and are found in the genes. Every characteristic that passes down from the parent to the child will consist of two alleles. The two alleles if appear to be same will be called homozygous. If they are not they will be heterozygous. The two parts of the allele are dominant and recessive.

In a situation with a dominant and recessive allele, the dominant will be expressive. Same is the case with two dominant alleles. The recessive will express itself only when two are present at the same time.

Some eye colours are naturally dominant while others are recessive. In a green vs blue combination, the green allele will be dominant. Brown will express itself over both green and blue and are considered to be the most dominating allele.

Let us consider some scenario of what the eye colour of the baby will be depending upon the colour of the parents.

  1. A mother has a blue allele while the father has a brown allele.

Ans. The child will have a brown eye colour since brown allele is dominant over all other eye colours.

  1. Both mother and father have blue eyes

Ans. The child is also likely to have blue eyes. Blue alleles are recessive and will be expressive only when combined together.

By making a chart which evaluates all the possibilities, we can easily predict the eye colour outcome of the baby. However, in reality, the process may be slightly more complicated. The gene heritance of eye colour is polygenic. One trait or characteristic is likely to be influenced by multiple other genes. For example, even with two brown dominant alleles, the baby may have a chance of not having a brown eye colour. Both the parents may be heterozygous and in that case, this outcome may be likely. Thereby, you may predict the outcome with high possibilities there is a still a chance of nonoccurrence. The room for unusual circumstances still remains.

The importance of melanin

The genes are also responsible for the amount of melanin that will be contained in the iris. A higher melanin production is linked with a darker eye colour. The melanin secretion starts once the baby is born and can continue till the age of three. It is this age when the permanent eye colour is determined. So, a child with a blue eye colour may have a different darker colour at the age of 3.

Two different eye colours!

Yes, people are also born with two different eye colours. It is likely to have both a green and a blue colour at the same time. It is rather unusual to happen and occurs in the very rare of the circumstances. The condition is referred to as Heterochromia. The disease is harmless and will unlikely yield any medical complications. It affects on average just 2-3% of the US population.

Samuel Mayer

Samuel has been part of Top Health Journal for quite a while now. He has immense interest in medicine and thoroughly enjoys writing about this theme. His philosophy is: write simple and short, always assuming your reader to be a layman who knows nothing about the topic. Twitter- @SamuelM54534849

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