Anyhoo, who has not ever encountered that situation where someone really impressed with something they learnt at school cannot really understand the possibility that in biology, as in so many other things in life, things are more complicated than they may seem, genetic inheritance one of them, as we’ll see today.
At school the most we did when dealing with genetics was to try emulate Mendel’s experiments with peas and multiply characters such as in AA x aa. However, when we try to understand the inheritance of a complex trait as eye colour this approach is simply not good enough.
A review in the Journal of Human Genetics explains it quite well. First, eye colour is the result of the differential expression of melanin in the melanocites of the iris, and there are at least (since the publication of this review last year more genes related to this effect have been found) 2 genes in chromosome 15 related to the quantity and quality of melanin produced; these two genes: OCA2 and HERC2 are tightly related. So much so, that the expression of one (OCA2) depends on the expression of the other (HERC2) because HERC2 contains the OCA2 promoter, a region indispensable for transcription initiation. This phenomenon of genetic dependence is called epistasis and causes the effects of one gene to be dependant on another. In our case the colour of the eye.
That’s why a change of base (from Timine to Citosine in the intron (gene region that doesn’t get transcribed and therefore doesn’t take part in the buildup of a protein) of HERC2 that contains the promoter for OCA2 inhibits partly transciption of OCA2 and produces a blue eyed person.
There are many more possibilities of producing colours, though. This very same gene, OCA2, has regions that specify several colours. One of these SNPs, rs1800407, specifically serves as a predictor of blue eyes since the changes it produces over the protein it codes for are extreme.
One of those things that seem impossible, specially considering what we learnt at school, is that two blue eyed people can have brown eyed offspring, and yet it is. Let’s go back to Mendel and his peas but this time let’s add what we’ve just been talking about, in this case you´ll see how it’s not only possible but plausible. From a father with a mutated HERC2 (h) promoter and a normal version of OCA2(O) that would mean brown but is in fact blue and a mother with a wild type version of HERC2(H) and a “blue” version of OCA2(o) the probability of brown eyed offspring would be up to 25%!
Those of you with green eyes might like to know that there’s a gene, MC1R, that’s called the redhead gene, because it’s commonly found in people with both phenotypes (red hair and gren eyes). Plus statistically green eyes seem to be the weirdest of all since there is no specific genetic code for it and it takes as much pigment as to produce brown eyes without being one of the “default” colours. Is then green the colour of envy or should we envy green eyed people?
One last fun fact. Remember Liz Taylor’s famous violet eyes? Hers were the result of a partial albinism, where some remaining melanin produces a hint of colour (blue) that combined with the red of the reflection of the light through the eye blood capilaries.
If you want to learn even more about this topic I recommend going through the article itself. Other than that just remember: brown doesn’t always beat blue. Because in genetics thigns are usually more complex than they might seem. Remember ENCODE and the foul name Junk-DNA?