So, I saw this show where this guy deduced that a girl’s claimed parents were not her biological mom and dad because they had blue eyes but the girl had brown eyes.

Ah, yes, you saw an episode of Jonathan Creek. A really good show, despite some occasionally ludicrous twist endings. And Jonathan Creek made an induction, not a deduction, but anyway.

Is this true? Can blue-eyed parents have a brown-eyed child?

The genetics of eye colour are more complex than meets the, uh, eye. You see, very little in genetics is actually straightforward. Researchers are still trying to fully understand how eye colour gets transmitted from parents to children.

Still? But we’ve sequenced the whole human genome!

Yes, we have, which is a brilliant achievement of the human race and should be celebrated. It is a major stepping stone in our increasing understanding of life and in the development of better diagnostic tools and medical treatments.

So how come we still don’t know which gene determines your eye colour?

Well, knowing every word in the dictionary does not mean you know what they all mean. We know where the genes are (more or less), but what they all do is still up for grabs. We used to think the inheritance of eye colour would be very simple; turns out, it’s not so simple.

So how many genes are involved?

We don’t yet know. There is one gene, OCA2, which seems to play an important role in determining eye colour. In fact, if you get your whole exome sequenced by a company like 23andMe (not that I’m suggesting you do), they will base their eye colour prediction on this particular gene.

Does this mean that a particular mutation in OCA2 will give you brown, or green, or blue eyes?

Well, in this case, we talk about a “polymorphism” because it is quite common and does not cause a disease. A polymorphism is a variation. And, in the case of OCA2, the determining polymorphism is not in the gene itself but outside of it.

If it’s outside the gene, how can it influence the gene?

The expression of genes is often regulated by stretches of DNA before and after the gene itself. It turns out there seems to be a regulatory polymorphism for the gene OCA2 which is found upstream of it. At this particular spot, you can have either an A (the DNA base adenine) or a G (the DNA base guanine).

One base decides my eye colour?

There are probably other polymorphisms in other genes which contribute to your eye colour, but here is what we know about this OCA2 regulatory polymorphism. If you get an “A” on both copies (the one you get from your mom and the one you get from your dad), you have an 85% chance of having brown eyes. This statistic was calculated for a European population, mind you. If you have an “A” on one copy and a “G” on the other, your chance of having brown eyes goes down to 56% while you have a 37% chance of having green eyes. With two “G”s, your most probable eye colour is blue, with a probability of 72%.

As you can see, this polymorphism alone cannot tell you what your eye colour will be. There are most likely other genes at play.

So was Jonathan Creek right in his deduction—err, I mean, induction?

In the episode “The Curious Tale of Mr. Spearfish”, he does say it is “most unlikely” for the girl to have brown eyes if both her parents have eyes the colour of the sky. Indeed, I would also qualify this scenario of “most unlikely”. Let’s look at the possibilities. If they have blue eyes, they probably have two copies of the “G” polymorphism, so their daughter must have two copies of the “G” polymorphism as well. “G/G” results in brown eyes only 1% of the time. The parents could also be “A/G”, but the chance of having blue eyes with this combination is 7% for each parent, or 0.5% for both. Unlikely. Perhaps the most likely scenario is that one of the parents is “A/G” (7% chance of blue eyes) and the other is “G/G” (72% chance of blue eyes), resulting in a daughter who happens to be “A/G” (56% chance of brown eyes).

Still, all in all, it is not a common scenario. Combined with additional evidence, one could infer the possibility that the child was not biologically theirs….

(Feature picture by GinjaSnap)