QotW - why are my eyes different colours?

Eva Higginbotham contacted David Mackey from the University of Western Australia to answer this colourful question...

Ellie - My name’s Ellie and I’m a teacher from Manchester. We’ve been learning about genetics and one of my students asked me a question that I couldn't answer..

Student - I'm a Year 9 student from Manchester and I've got one blue eye and one brown eye. When I have kids, are they going to have a blue eye and a brown eye like me?

Eva - Now I am fully invested in finding out the answer to this question because I also have eyes which are different colours - the iris of my left eye is light green, whereas my right eye is split down the middle - half green and half brown. And when I asked my science teacher about the genetics of this when I was in year 7, she called me a mutant! So I’m glad your teacher Ellie is more supportive of this excellent query... I went ahead and put the question to eye colour expert David Mackey at the University of Western Australia...

David - Iris colour, and similarly hair and skin colour, is mainly influenced by the production of pigment which is predominantly melanin. A brown-coloured iris has lots of melanin, while a blue coloured iris has little melanin. The eye appears blue for the same reason the sky appears blue, from the scattering of light. In the case of an eye, light is scattered by the clear collagen fibres in the front part of the iris.

Eva - Similarly, there’s no ‘green pigment’ for green eyes, instead the eye appears green due to a combination of minimal melanin and light scattering. So that’s why eyes appear different colours, but what determines what colour eyes each of us have?

David - We used to think the genetics of eye colour were quite straightforward, with brown being dominant. We now know that the genetics of eye colour are quite complex with dozens of genes and environmental factors also playing a role. Newborn babies often have lighter-coloured eyes and the pigment increases over the first few years of life. Having eyes of different colours is known as heterochromia and this is usually due to environmental factors. David Bowie and eye injury

Eva - So if heterochromia is often caused by environmental factors, what does that mean for the children of people with heterochromia?

David - It is unlikely that your children will have eyes of different colours. However, if you were a dog, there is a high chance of your offspring having heterochromia. In dogs, and some other animals, just as their fur can have patchy areas of pigment, they can have one pigmented and one less-pigmented iris. This is particularly noted in Arctic Huskies. Some genes associated with this have been identified.

Eva - So there you go, it seems that it’s unlikely that your children, or my children, will also have beautiful differently coloured eyes - we’re the lucky ones! We’ll just have to get them a Husky instead...Thanks David. Next week, we’re answering this question from Rob

Rob - Is dark matter lumpy or like grains of sand?