What makes different shades of blue eyes?

Question

I know there are very little or no traces of melanin in blue eyes, and I'm assuming that is for light to dark blue eyes. So does anyone know what makes individuals have different shades of blue eyes? (or for any color of eye for that matter)

Answer 1

You're right about the melanin. The more melanin, the darker the eye color, so brown eyes have LOTS of melanin.

Eye color is a trait that is controlled by multiple sets of genes. (On the other hand, some traits are only controlled by only ONE set of genes.) Each set of genes that controls eye color may call for a different amount of melanin. This is why all blue eyes are not the exact same shade of blue, and ditto for green, brown, hazel, etc. It is even possible for one person to have two different colored eyes.

The same can be said for skin and hair color as well as height - these characteristics are also controlled by multiple genes.

Answer 2

OCA2 (syn. bey2, EYCL3) codes for a protein that helps give hair, skin and eyes their colour. OCA2 was thought to have dominant and recessive traits for brown and blue eye colour. The iris is colored by, its background skin, its stroma, and the stroma’s density. Pigments localize in the iris's stroma. The brown version of OCA2 exists in the stroma, the blue version does not. Either exist in the background skin.

Mutations in OCA2 cause the most common type of albinism more common in Africans than in Caucasians. So far 3 point mutations show up in alleles for this gene that are linked to blue eye colour and 1 linked to green. These alleles are also tied to skin tones, freckling and skin moles. Therefore this pigment is expressed elsewhere. The stroma no longer express this protein if a nonbrown allele is inherited but other cells express this pigment so the person is not albino.

So if the gene is knocked out there is no pigment protein. If the gene is slightly different the color value changes so this mutation may be to regulate, to produce more or less protein. Last, the stroma’s cells are able to express only the brown allele.
The 4 point mutations occur in the 5' regulatory sequences that bind transcription promotors. The several mutations may allow different rates of transcription in cells capable of recognizing the promotor binding sequences but the stroma's cells no longer recognize the sequences at all. Stroma's must use a different promotor protein.

There are at least two other theoretical genes, referred to as EYCL1 & EYCL2, which effect eye color.
The other reason for different colours is in the organization of pigment in the iris. Iris' can be light near the pupil and dark at the outer edge or even have different colors on the inner and outer edges.