The complicated genetics of human eye colour inheritance

I’ll be taking a break from blogging over the next month as the “egg” (or is that the chicken?) will be hatching.  As you do, when about to have a baby, I’ve been thinking a bit about inheritance lately – what colour eyes or hair will my baby have, how tall, who will he/she take after? The questions are endless really (and no we DONT know the sex!). 

Being a geneticist, I figured the answers to at least some of these questions must be relatively well worked out.  Eye colour for starters – we all know brown eyes are dominant to blue, right?  And if you google “eye colour inheritance” you can find any number of “eye colour calculators” that will work out the likely eye colour of your offspring.  I tried to use one of these and immediately ran into a problem – even the most sophisticated one I could find only allowed brown, blue or green as eye colours.  Well, my eyes are hazel (grey/green with a brown ring around the pupil).  Does this count as green? And my partners eyes are not exactly blue or green, they are kind of greyish-greenish-blueish with a tendency to change colour depending on his clothes and the light.  So having fallen at the first hurdle, I began to suspect that eye colour might be a whole lot more complicated than what you learn at school. 

And it turns out that it is – eye colour is a polygenic trait, which means there are multiple genes that interact to produce the colour.  How many genes there are, where they are and what they do exactly is only partially known. 

Eye colour is determined by the amount and distribution of melanin in the iris.  Brown eyes contain more melanin than green eyes, while blue eyes have very little melanin.  The old textbook explanation that brown is dominant to green and both are dominant to blue does generally hold true, but not always, and is too simplistic to explain the multitude of variations on grey, blue, green and brown.  Geneticists have been modelling the inheritance of eye colour since the late 19th century, but originally described inheritance along the simple Mendelian dominant/recessive lines I described above.  It didn’t take long for exceptions to this rule to become apparent (like two blue eyed parents having a brown eyed child) and it became obvious that there must be more than one gene involved. 

In the 1980s chromosome mapping techniques were developed which enabled researchers to identify particular chromosomal regions (or loci) that are associated with inheritance of particular traits.  A locus associated with green eye colour (named Gey) was mapped to chromosome 19, and a locus associated with brown/blue eye eye colour (named Bey) was mapped to chromosome 15.  These “loci” are not genes as such, but they are regions of the chromosome that contain the genes likely to play a role in eye colour.  Many of the textbook explanations (and online calculators) for eye colour will tell you that there are 2 variants (alleles) for each of these loci - a green (dominant) and blue (recessive) allele at Gey, and a brown (dominant) and blue (recessive) allele at Bey.  This is a useful model for demonstrating how inheritance of a polygenic trait works, and in the case of eye colour it does explain some common patterns of inheritance.  But there are clearly additional alleles and additional genes at work that are only now beginning to be identified. 

The Bey locus has now been identified as a gene called OCA2.  This gene codes for a protein that stimulates the melanin-producing cells in the eye to mature and well, produce melanin.  OCA2 now looks to be the major determinant of eye colour, with some recent research estimating that this gene alone is responsible for about 74% of the variation in human eye colour.  There are far more than just two variant forms of this gene – dozens of alleles have now been identified, many differing by only a few changes in their DNA sequence.  A 2006 study found that some of these changes are highly diagnostic for particular eye colours – in fact, eye colour can be predicted reasonably well (but not entirely) from a person’s genotype at OCA2.  With the availability of human genome sequences, more new genes associated with eye colour are being discovered, but how these genes interact with OCA2, and exactly how much they contribute to eye colour is yet to be determined.   

So what colour will my baby’s eyes be?  After all my research I’m really no closer to an answer, but I’m guessing probably some variation on green.  I’ll be sure to let you know.

References (these appear to be free to access):

Sturm RA and Larsson M (2009) Genetics of human iris colour and patterns. Pigment Cell and Melanoma Research 22: 544-562. DOI: j.1755-148X.2009.00606.x

Duffy DL, Montgomery GW, Chen W, Zhao ZZ, Le L, James MR, Hayward NK, Martin NG, Sturm RA (2006).  A three single-nucleotide polymorphism haplotype in intron 1 of OCA2 explains most human eye-color variation. American Journal of Human Genetics 80: 241-52. DOI: 10.1086/510885

 

9 Responses to The complicated genetics of human eye colour inheritance

  1. [...] colour inheritance update #1 To update my post on the inheritance of human eye colour (as I’m sure you are all dying to know), the little monkey was born with dark grey eyes.  No [...]

    • April says:

      Has your babies eyes changed any? I have brown eyes, my husband has light green. Our son was born with gray/blue eyes that every once in a while looked green. Now, at 9 months, his eyes are a dark green with a hint of gray and don’t seem to be changing.

      • hilaryml says:

        The munchkin’s eyes are still grey/blue and don’t seem to be changing and she is now 11 months. I was wondering if they would develop a greenish tinge as she got older, but it doesn’t look like thats going to happen. Interesting to know that your son has developed his green eyes by 9 months…

  2. As an actor who has eyes which change colors (much like you described your partner’s eyes) this has been a real bother for me. I would like to call it a “challenge” except that the term would be misleading since it implies there is something I can potentially “do about it” if I try hard enough… but the fact is, I came across this article while searching for a description of “hazel eyes” that might help me to determine whether that was closer to being descriptive of my eye color than Black, Blue, Brown, Grey, or Green, which are the other choices available to me as I attempt to set up my profile on http://www.us.castingcallpro.com and of course am faced to work within the limitations of the system… and they actually have 6 choices available which is an unusually high number of eye color choices as such sites go.

    Thanks for the description, by the way, of Hazel eyes. I’m still trying to decide. Wikipedia has a nice article on eye color, and they even have a picture on the page that looks almost exactly like what my eyes look like right now… but it’s labeled “Close up of a human Iris” rather than saying what that color combination is called… although it fits the description of Hazel better than Black, Blue, Brown, Grey, or Green, which I guess is why I went looking for more information. It’s not that I didn’t have a clue… but rather that there is no simple answer.

    By the way, my mood and emotions play a small role in determining my eye color also, presenting a further issue…. but that is at least something I have a degree of control over, so…. that’s a challange. :)

    I’ve run into the same problem when it comes to questions like “ethnicity” which “mixed” is about the only thing I “fit well” into but that’s not very descriptive and can exclude me from consideration for roles that I could easily pass for. I honestly feel in filling out the forms on such sites like I am reducing my acting talent down to that of a mannequin for the purposes of listing myself as available for work as an actor… but I understand that those who set up such sites have an impossible task which they must reduce in complexity to the point of being doable so that human beings can be catalogued in a way that allows a casting director to search for something specific. I will be SO HAPPY when I am finally well enough known that people look for ME rather than having to find me on the off chance that of all the poorly-descriptive options I had to choose from in describing myself, I happened to pick the ones they also picked to describe what they are looking for. For now… I have to work with what’s available and figure it out as I go.

    Donald Arthur Kronos – Actor

    P.S. – If anyone reading this wishes to reach me, there is contact information about me available through IMDB PRO or simply message me on FaceBook.. not on the wall, but through the Message feature. Don.

  3. nila says:

    its a challenging research how much u can predict eye colour of a unborn child through his/her parent’s eye color

  4. Angelica says:

    Following up on your child…Are her eyes still gray/blue? Based on my experience with my children I can say that sometimes eye color keeps changing until the child is 5.

  5. Pretty Baby! says:

    Pretty Baby!…

    [...]The complicated genetics of human eye colour inheritance « The chicken or the egg[...]…

  6. fasha says:

    ok i got one for u. most of my mothers side is blue eye. that being they have the blue with the yellow in the center called usualy Central heterochromia. while on my fathers side they have the hazel that is the brown on the iris side and the green on the out. my husbands side all were brown eyes.
    when my daughter was 3 weeks old her left eye turned to brown. i did some research its called Heterochromia iridum now the question being is that this. since being that its a semi rare event type thing how is this played into gentics. although im suspecting that it has to do with my mothers side because of the central heterochromia.
    we had thought that she would have either the hazel or brown eyes because it is more doment then the blue. did not even think that the 2 colors would be out. so any ideas on what this is?

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