Why are grey eyes rare?

Genetics of grey eyes:

Grey eye colour is caused by a combination of genetic factors, including the amount and distribution of melanin in the iris. Melanin is the pigment that gives colour to the skin, hair, and eyes. Different combinations and concentrations of melanin can result in different eye colours, including grey. While we generally talk of brown-eyed, blue-eyed, and green-eyed people, most of us actually have some combination of these colors.

Low melanin content:

Grey eyes typically have a lower amount of melanin compared to other eye colours, particularly the brown pigment eumelanin. Lower levels of melanin lead to reduced absorption of light, causing light to scatter and reflect differently, giving the eyes their distinctive grey appearance.

Rayleigh scattering:

The physics behind grey eyes involves a phenomenon known as Rayleigh scattering. When light interacts with the particles in the iris, shorter wavelengths (blue light) are scattered more efficiently than longer wavelengths (red light). This means that more blue light is reflected back, giving grey eyes their bluish-grey hue.

Genetic variations:

Apart from the quantity of melanin, genetic variations in genes responsible for eye colour can also contribute to the rarity of grey eyes. For instance, mutations or variations in the OCA2 gene, associated with producing the P protein for melanin synthesis, have been linked to grey eye colour.

Common in certain populations:

While grey eyes can occur in individuals of different ethnicities and regions, they are more prevalent in certain populations. For example, grey eyes are relatively more common in Northern and Eastern Europe, including countries like Iceland, Scotland, Estonia, and Finland.