How does eye colour come about?

Anatomy & Physiology

The colored ring of our eye/eye color is called iris (rainbow skin). The iris consists histologically of several layers. The layer that is decisive for the eye colour is called stroma iridis, where stroma means connective tissue.

This layer consists mainly of collagen fibres and fibroblasts, i.e. cells that produce components of connective tissue. In addition, this layer contains the two muscles responsible for the width of the pupil. These are on the one hand – Musculus sphincter pupillae, which constricts the pupil, and on the other – Musculus dilatator pupillae, which is responsible for the dilation of the pupil).

Eye colour – What is behind it?

Another cell population is decisive for eye colour: the melanocytes. They produce the dye melanin, which is also of decisive importance for the colour of skin and hair. People whose iris contains few melanocytes have a lighter eye colour than those who contain many melanocytes.

So people who have very few or no melanocytes in their iris have blue eyes. But when the blue colour is created, there is still a lot of discussion. There are two main components responsible: 1. the pigment epithelium located directly behind the stroma iridis (myoepithelium pigmentosum, attention, this should not be confused with the pigment epithelium of the retina, which has a different function).

If the retina shines through the iris almost unhindered, the iris appears blue. 2. How unhindered the pigment epithelium can shine through is again dependent on how much collagen is stored in the stroma iridis, because the collagen content determines how much light is scattered and reflected and this in turn is decisive for the impressive eye colour in the end.

But what about the eyes that are not blue? If melanocytes are occasionally stored, the iris appears green or grey. If there are numerous melanocytes in the connective tissue layer, the iris appears brown. How the innumerable color facets and shades that exist of each of these colors are created, still remains a small mystery for which there are many hypotheses.

Inheritance of eye color

For a long time, the Davenport model was considered to be the written model here. It was based on a single gene for the inheritance of eye color. However, it is now clear that the mode of inheritance of eye color is polygenic.

This means that more than one gene is responsible for passing on eye colour from parent to child. Some eye colours are more dominant than others. Brown is the most dominant of all eye colours, followed in descending order by green, blue and grey.

Theoretically, if the father has brown eyes and the mother has blue eyes, brown will prevail over blue and the child of both will have brown eyes. However, it is not quite that simple, since there are two alleles of each gene. For example, the father who has brown eyes (phenotype) may have one allele for brown eyes and one for blue eyes in his genetic material (genotype).

He passes only one of the two alleles on to his child. So a child of a brown-eyed father does not necessarily have to have brown eyes. But that is not enough.

Further genes complicate the genetics around eye colour many times over. Most babies of European descent are born with blue eyes. The reason for this is that the iris of newborns does not yet contain any pigments.

The iris is only coloured by melanin, an endogenous dye that reacts to light. After birth, melanin is hardly present. The colour of the eyes is determined by the genes of a person and can change in the first year of life depending on this.

As a rule, the final eye colour of a person appears 3 to 6 months after birth. A simple examination of the newborn’s iris can give an indication as to what will most likely be the basic colour of the eye: if you look at the iris from the side under simple fluoroscopy, you can see high or low levels of melanin. If the iris continues to appear light blue with this method, it means that there is no melanin.

In this case the eye colour is most likely to remain blue. However, if the iris shimmers golden, this indicates a certain amount of melanin, and the iris will probably still be brown or green in this case. In newborns from Asia, Africa or Latin America the eye colour at birth is often mostly brown.

Sometimes it happens that even after the first year of life the eye colour of a person changes. It has been found that hormonal fluctuations or biochemical processes in the body can have a changing effect on the iris. For example, hormonal influences can very rarely cause changes in eye colour during puberty or pregnancy.

In a study among pairs of twins, it was found that in about 10% of fair-skinned people, the colour of the iris changes during the course of life. However, if there is a rapid change in eye colour, an ophthalmologist should be consulted to rule out disease as the cause. This could be an inflammation of the eye, for example. In addition, an injury to the optic nerve can also cause the colour of the iris to change.