Blue Eyes, DNA, and Geneology

Ten years ago, a paper was published that discovered the genetic mutation that causes blue eyes.  They also concluded that it seems that the mutation only occurred once in human history, and that anyone with blue eyes is descended from that person.

Comments I have seen online on the topic show that some people get confused by this.  Some think that means they aren't related to people without blue eyes, while others find it hard to understand that we are all related.  Having blue eyes myself, the topic is interesting to me.  I also find it interesting to see how closely the human family is related, so I want to talk in more detail about that.

So first off, a bit about genetics and blue eyes.  Humans have 23 pairs of chromosomes with 3.2 million base-pairs.  About 98% is non-coding DNA and the remaining 2% make up some 20,000 protein-coding genes.

On the 15th Chromosome is a gene called OCA2.  This gene produces P protein, which is used in making melanin.  A deformation of this gene is what causes albinism.  The gene prior to this one is called HERC2, and it was found that a single base-pair change in that gene acted like a switch and reduced the production of P protein to cause blue eyes.

From what I gather, it seems like the original study was just trying to identify what causes blue eyes, but they found something really interesting.  Other colors of eyes have great variation in their genes.  Those with blue eyes, besides the one base-pair change, also had 6 other identical base-pair changes in their HERC2 gene.  These individuals came from Denmark, Turkey, and Jordan.  You would expect that if the same mutation happened different times in human history, it would mutate the gene in different ways.  Since the mutation was identical, that suggested to the scientists that it was caused by a common founder mutation.  They believe that all humans alive today with blue eyes inherited that gene from that individual.

I don't have access to the full article, but news reports say they estimate the mutation to have occurred some 6,000-10,000 years ago, probably near the black sea.

One common misunderstanding arises when the above is summarized as "people with blue eyes all descend from a common ancestor."  While true, that isn't what is interesting (after all, we all are—I'll get to that later) it is interesting that blue-eyed people all inherited the same gene from the same individual.  Anyway, when it is summarized that way, people sometimes think blue-eyed people are in their own family, all related, while those of other eye colors are unrelated.  This simply isn't the case and is easily disproved with a simple punnet square.

Eye color is influenced by more than one gene, but let's just make things easy and pretend it's only the HERC2 gene, and there's just brown and blue eyes.  Brown eyes are dominant-- just one brown-eye gene will cause the production of P Protein and eyes will get their melanin.  It takes two blue-eye genes to prevent that.

A blue-eyed person can marry into a brown eyed family, and they can carry that gene as a recessive gene for generations until they meet up with someone else with a recessive blue-eyed gene.  Now there's a possibility that they can have a child with blue eyes.  They are related to their parents and siblings, even if they all have brown eyes.  And vice-versa.

An interesting observation in the punnet square-- we all know that you inherit half your genes from each parent, who get half from their parent, and so on.  You can clearly see which grandparents the blue-eyed child received their genes from.  Since you only have one pair of each gene, the interesting part is that you can also see that the other two grandparents contributed no genes to the blue-eyed grandchild.

This is true of all our genes, half come from each parent, who get half of their genes from their parents, and so on.  We only have one pair of each gene, so each individual gene can only come from two of our parents, grandparents, great-grandparents.  It becomes less and less likely that any one gene came from any one ancestor.  And you only have so many genes, so it is not only possible, but a mathematical necessity that you have some ancestor that you inherit no genes from.

As I said earlier, we have about 20,000 protein-coding genes.  This is less than 2¹⁵ so 16 generations ago if each ancestor wanted to contribute one of those genes to you, there would not be enough of them.  16 generations is far enough back to guarantee that you have some ancestor that contributed nothing to your genes.

But that was just protein-coding genes.  Surely your non-coding DNA still contributes to who you are-- not only as influencing other genes, but even solely acting like an identification marker.  Even though most of our DNA is the same, let's assume that it can all be different.  And let's assume instead of inheriting chromosomes or genes, we can inherit single base-pairs, fully at random.  3.2 million base pairs is less than 2³² so it would only take 33 generations to guarantee that you have some ancestor that contributed nothing to your DNA at all.  If you estimate a generation as 25 years, then 40 generations is only 1,000 years ago.

However, going back 2⁴⁰ generations would make a trillion ancestors.  There has never been a time on earth when a trillion people were alive all at the same time.  So the reality is that after many generations, relatives will intermarry.  Usually very distant relatives.  In that way, if you draw your family tree 40 generations back, you find lots of duplications.  People who are more duplicated on that tree than others are the ones that have a stronger likelihood of influencing your DNA.

And that brings me to the third interesting thing I want to talk about here.  Typically people marry those that live close to them.  More so especially in historical times than now.  But it only takes a few people moving around to cause descendants to spread in area over the course of many generations.  Mathematical models show that it likely that our most recent common ancestor, someone from whom all mankind can find on our list of ancestors, lived only a few thousand years ago.  And just a few thousand years before that, if everyone today made a list of all our ancestors living then, we'd find our lists identical.  Just a few thousand years ago, everyone alive is either the ancestor of everyone alive today, or no one alive today.

I think that's really interesting.  Sometimes we think of our common ancestors living in Africa a hundred thousand years ago.  But the reality is that there is always intermixing, and we have common ancestors in the much more recent history of just a few thousand years.  Whatever race you are, you can count among your ancestors members of all other races.  People naturally look for differences, the "us" vs. "them" but "us" is really all there is.  Thousands of years somehow cause us to forget that.

And that works forward too-- Some day thousands of years from now, you and everyone you know will also be either the ancestor of everyone, or no one.