How does eye color gene work?

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If my paternal great grandmother had green eyes but both my grandparents, parents and myself have brown eyes, what are the chances that my baby will have green or blue eyes if my partner had green eyes or blue eyes? And do I still carry the green eye gene?

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Blue eye color is a recessive gene. Unless one of your parents had it and passed it on to you and the same for your mate than the chance is 25%.

You’d both have to carry the gene. Blue eye color isn’t expressed unless you inherit both copies from both parents.

Two genes: OCA2 and HERC2 are the primary determiners of eye color. Several other genes (ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, and TYRP1) have a smaller effect, on the intensity of the coloration of the iris (and skin and hair). While they don’t change the color, they influence the many other shades of eye color in the blue vs hazel vs brown system regulated by OCA2 and HERC2.

Every person has two copies of all these genes, one from each parent. This gives a lot of possible permutations of color, as shown in a table like [this one](https://www.edow.com/general-eye-care/eyecolor/).

There are two different pigments that control eye color: yellow (pheomelanin) and brown (eumelanin). Lack of any pigment leaves the eye looking blue. There are different genes that control these two pigments so it’s not a simple equation of one gene in one location. it’s multiple genes, which makes determining what your kid will be like difficult. If you have brown pigment, your eyes will be brown. If you have yellow pigment, then the yellow combined with the blue of no pigment, make green. So there’s no one gene for green eyes–you have to have the right combination of pigments, which will include genes for pheomelanin and genes for no eumelanin.

Eyes rarely have just one pigment uniformly distributed in the eyes, though. More often, people have a combination of pigments and they’re scattered throughout the eye. So you end up with people with combinations of colors. Like with hazel eyes they may look green on the outer part of the iris but near the pupil they’ll look brown. With amber eyes, they may have some yellow and some brown. Etc. So you know that you have genes for brown pigment because that’s what your eye color is, but you might also have genes for yellow pigment and you could also have a gene for no eumelanin or no pheomelanin. If you and your partner pass on all of the genes for no pheomelanin and no eumelanin, then the baby will be blue eyed. We don’t know if you have such genes, we only know what the expression of the genes you have is. It’s possible you didn’t inherit the genes that led to your paternal great grandmother having green eyes, and you might have 2 genes for brown eyes, which means your kid will definitely get the gene for brown eyes. Or you could have the gene for brown eyes (eumelanin) from one parent, and also genes for pheomelanin but the brown pigment obscures the yellow pigment so you don’t know you have it. We simply don’t know.