My brown-eyed girl?

GCSE double science has a lot to answer for. It told me that if I found a blue eyed man and if we loved each other very very much and had a baby that with my blue eyes, two recessive genes would make a recessive gene and our babies would have blue eyes. Visiting Fred’s sisters as part of the grand baby parade, his sisters’ bet was on Elphie’s eyes turning brown from their current dark blue / slate colour – leading me to wonder, could they be right? Can two blue eyed parents produce a brown eyed kid? Had GCSE double science been fooling me all along?
I needed to get an answer on this before Fred started questioning his parentage and looking strangely at the brown-eyed postman. Although in reality I think it should be the other way round – I can see definite signs of Fred’s features in her, she has his legs and feet (just the features a daughter wants from her father) and the dark colouring would seem to come from his side of the family, but where the signs of me are in the genetic mix are less clear. If she hadn’t done a stretch in my womb and come out of my ying yang, I would be the one questioning the parentage!

The colour of your eyes relates to the amount of melanin in your iris – the greater the melanin the closer to brown eyes you get (with green and hazel eyes somewhere along the way). As What to Expect explains, caucasian babies are born with dark blue or slate grey eyes and start producing melanin in their irises from birth which will then darken the irises due to exposure to UV light. However, it will typically take nine months to develop a stable colour, sometimes it will take years.

As my oracle on this, Stanford at The Tech Museum of Innovation, explains:
“There are two different layers in the iris, a front and a back one. In between the two layers is an area called the stroma.

The stroma is a clear tissue with many proteins floating around in it. One of these proteins is called collagen. We will get back to that later.

Almost all eye colors have a lot of melanin on the back layer of the iris. Most differences in eye color come from the amount of melanin made on the front layer.

For example, both brown and blue eyes have lots of melanin in the back layer. The difference is that brown eyes also have a lot of melanin in the front layer and blue eyes do not. So brown eyes are dark because lots of melanin in the front layer absorbs any light hitting the iris.

In blue eyes, though, light can pass right through the clear front of the iris and reflect off the melanin in the back of the eye. But the light doesn’t reflect back through a completely empty stroma. The collagen in the stroma gets in the way of the light on its way back to the front of the eye. When the light hits the collagen it bends and looks blue.

It is just like the sky. The sky is dark in space but when we look at it from Earth during the day, it looks blue not black. This is because light from the sun hits particles in the atmosphere and reflects blue. This effect is called Rayleigh scattering.”

So for now her eyes look blue, so far so expected, but could they end up brown like Fred’s sisters predict?

It seems that eye colour is far more complicated than GCSE biology would have me believe with at least 16 genes affecting eye colour.

But the ability for two blue-eyed parents to produce a brown-eyed daughter can be demonstrated by another excellent article by The Tech Museum of Innovation. The two genes that have the most influence on this issue are OCA2 and HERC2 (the gey gene has a broader influence on eye colour but for us to have blue eyes I think we would both have to be recessive in this). Both OCA2 and HERC2 need to be working for melanin to be produced, if one of them is broken then none will be produced and the eyes will likely tend towards green or blue depending on the gey gene.

This diagram from the article explains it better than I can:

And the science bit from the article:

“OCA2 is one of the key genes in determining how much pigment gets made. So it makes sense that if both OCA2 copies are broken, someone would have blue eyes. Because they can’t make a lot of pigment.

Most of the HERC2 gene has very little to do with eye color. There is one small section in the middle, though, that controls whether OCA2 is turned on or not…

Think about OCA2 like a light bulb and HERC2 as a switch. If the light bulb is burned out, it doesn’t matter if the switch is turned on. Just like it doesn’t matter if HERC2 works in someone with broken OCA2. Flipping the switch to a burned out bulb won’t give you any light!”

Generally people get two copies of most genes – one from their father and one from their mother. So my blue eyes could be caused by a pair of broken OCA2 genes provided by my parents combined with broken or working variants of HERC2 (once OCA2 is broken then that’s it remember). Or it could be the other way round with both of my HERC2 genes broken.

The way I could produce a brown-eyed child is if both Fred and I pass on working OCA2 and HERC2 genes which will dominate over any broken gene they are paired with (you need both the genes in the pair to be broken to get blue eyes).

An interesting edge case is if my blue eyes are caused by broken OCA2s but my HERC2s are both working, and if Fred is the opposite with both OCA2s working and both HERC2s broken, then I will pass on a broken OCA2 and a working HERC2 which combined with Fred’s working OCA2 and broken HERC2 will mean there is a working variant in both OCA2 and HERC2, so if you were reliant on just these genes alone then all our children would have brown eyes.

Crazy stuff this genetics – no wonder them dumbed it down at GCSE!

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