• Question: why do we have different eye colours?

    Asked by ands493had on 27 Nov 2023.
    • Photo: Samantha Slater-Lewin

      Samantha Slater-Lewin answered on 27 Nov 2023:

      The colour of the iris in the eye, like skin and hair colour, is determined by the genes you get from each biological parent. The genes determine how much of a pigment called melanin is produced. Melanin is also what gives each person their hair and skin colour. The more melanin in your genes, the darker the colour.

      We have different eyes colours because the combination of genes passed from each parent down to their offspring is unique (apart from identical twins). That is why you will see people in the same family with different eye colours, hair colours and skin tones. It’s really interesting how genetics works.

    • Photo: Michael Schubert

      Michael Schubert answered on 28 Nov 2023:

      Samantha has covered a lot of the answer! Eye colour can be quite complicated because, although we used to think it was controlled by just one gene, we now know that multiple genes are involved. There are also factors outside your genes that can contribute to eye colour; for instance, the way different genes or proteins interact with one another – called “epigenetics” – can also play a role. Even some illnesses and injuries can affect eye colour!

      Did you know that blue eyes actually don’t have any blue in them at all? The blue look comes from the way a structural protein in the eye (collagen) scatters light – the same reason the sky looks blue. Grey eyes also don’t have any coloured pigment, but because they contain more collagen than blue eyes, the colour we see is different.

    • Photo: Walter Bodmer

      Walter Bodmer answered on 28 Nov 2023:

      Because there are variants of genes controlling eye colour in our populations. Not clear what advantage these might have given in the past, which is what would have led to selecteion for different variants in the human population.

    • Photo: Caroline Hyde

      Caroline Hyde answered on 28 Nov 2023:

      I feel others have answered the ‘how it works’ part of this question really well. However, it is an interesting question in terms of evolution. Does having different eye colour provide an evolutionary advantage? Perhaps we can look to animals for this. We know that some cats or dogs have unexpected eye colours, so perhaps eye colour is just the result of spontaneous mutations that over the years have expanded our pool of (possible) colours? What were your thoughts?

    • Photo: Martin Minarik

      Martin Minarik answered on 28 Nov 2023:

      Caroline’s question is really interesting! Most humans have more or less dark brown eyes, and this was certainly the eye colour our African ancestors had. Together with dark skin, there was strong natural selection for this to protect us from strong UV light while our ancestors lived in Africa, and they took this with them through south Asia to Australia as well. However, as humans migrated north to colder climates, this selection relaxed. In fact, it actually became useful to become light-skinned to be able to make enough vitamin D from the miserably weak sunlight we were getting up here. Eye colour selection might have become more relaxed at the same time as well, allowing for mutations responsible for a greater diversity of brown shades and other eye colours.

      In the case of blue eyes, there was apparently a random mutation somewhere on the way from Africa to Europe, which then spread widely amongst Europeans. Not sure why it was so successful. One reason surely is that up north it was not harmful in any way anymore, so it wasn’t selected against. But we aren’t sure why it spread so much. One idea is that people might have found it attractive so it affected how successful blue-eyed people were in finding partners (I’d definitely use different criteria for finding a partner, although thinking of it now, my wife has sort of bluish eyes :D). I’ve even read an article that speculated that blue eyes let more light in so people get less depressed during gloomy winter months 🙂

    • Photo: Ping Zhang

      Ping Zhang answered on 1 Dec 2023:

      The genetic determinants of eye colour involve multiple genes including OCA2 and HERC2 encoding proteins that are responsible for cell pigmentation, whoes genetic variation- ​single Nucleotide Polymorphisms (SNPs) are therefore the primary predictor of different eye colours.