If blue eyes are regressive (overridden by brown eyes) then, how could millions of north Europeans end up with blue eyes, supposing that blue eyes come from a single mutation?
In: Biology
Even if an entire generation, or several generations were not to have brown eyes, the recessive gene would remain in the gene pool and could come up again. It’s likely that the first ever human that had blue eyes didn’t have any blue eye children, or even grand children. It probably took multiple generations of breeding within a community, or several communities before the gene became common enough that another blue eyed child was born.
From an evolutionary standpoint there was something about the people with blue eyes that made them successful and made a lot of children spreading that particular gene. Even if it was a case that they just happened to survive a particular catastrophe leading to the blue eyed descendants being more common place.
For all the people with blue eyes, there’s a lot more people with the recessive gene.
If human dating and paring were truly random, then blue eyes would be a lot less common and would be at risk of going extinct, but pairing is **not** random.
People do tend to date people with similar ethnic background, so the chances of a pairing of people with the recessive gene is higher than it would be if dating were random. Asian people for example make up more than half of the overall population. So if Europeans dated randomly they would be far more likely to date an asian person that is highly unlikely to have the gene so the gene would become less common. But while European + Asian pairings are somewhat common, European + European pairings are far more likely if you get the point.
The blue eye gene is now so common in the European population, that even if you have brown eyes, depending on the ethnic background of you and your partner, there can be a reasonable chance that you can have children with blue eyes.
Blue eyes are also considered desirable by a lot of people, so that can encourage people to date people with blue eyes to pass on that gene. If it was undesirable it would be less likely to be passed on.
They key here is that eye color isn’t an entirely independent trait. Blue eyes is the lack of melanin, so any genes affecting melanin production in general will also affect eye color. People living in places with less sunlight need to have lighter skin (aka less melanin) for better vitamin D production, and lighter eyes happen as a side effect
Even if it’s a recessive gene, the survival benefits means it’ll spread through the population. High melanin genes can’t dominate if the people are dead because of vitamin D deficiency.
https://en.wikipedia.org/wiki/Eye_color#Genetic_determination
> OCA2 Associated with melanin producing cells. Central importance to eye color.
> HERC2 Affects function of OCA2, with a specific mutation strongly linked to blue eyes.
https://en.wikipedia.org/wiki/HERC2#Protein_function
> SNPs of HERC2 are **strongly associated with iris colour variability in humans**. In particular, the rs916977 and rs12913832 SNPs have been **reported as good predictors of this trait, and the latter is also significantly associated with skin and hair colour**.
https://en.wikipedia.org/wiki/Vitamin_D#Deficiency
> Dark-skinned people living in temperate climates have been shown to have low vitamin D levels.[32][33][34] Dark-skinned people are less efficient at making vitamin D because melanin in the skin hinders vitamin D synthesis.
This is one of the myths of recessive genes. The idea that they will “go extinct” is not accurate, since they are just as likely to be passed down as the dominant genes, just less likely to be expressed.
It comes from the fact that there are dominant and recessive genes. In eye colour the dominant gene* is for brown (B) and the recessive is blue (b). You get two copies of the eye colour gene, one from each parent, and you pass on one of yours to each of your children. If you get two dominant browns (BB) you have brown eyes, if you have two recessive blues (bb) you have blue. But if you have one of each (Bb), the dominant gene is the one you manifest so you have brown eyes. But importantly if you have both genes, the brown gene doesn’t destroy the blue one, and you could pass either one on to your children. So you might have the “blue eye gene” but not blue eyes.
*(a gross oversimplification – there are many genes which decide eye colour and the model of dominance and recessiveness is much more complex, but it’s a good enough simplification)
If you imagine two societies which meet, one where everyone has only copies of the blue eye gene (everyone is bb) and one where everyone has two copies of the brown eye gene (BB). They mix and interbreed until they are one group, indistinguishable from one another. Imagine (this would not actually be the case as we will show, but imagine) that we get to a point where every single person has one brown eye gene and one blue eye gene (Bb). Everyone has brown eyes. Now lets look at the children of any two of these Bb people:
Every child has a 50/50 chance of getting either a B or a b from their father.
Every child has a 50/50 chance of getting either a B or a b from their mother.
So 50% of the kids get the brown eye gene from their father and so have brown eyes, we can ignore them for now.
Of the remaining 50% who get the blue gene from their father, 50% of those (25% total) get the brown gene from their mother (so are Bb) and have brown eyes.
The remaining kids get a blue gene from their father and a blue gene from their mother (bb) and so have blue eyes even though both of their parents’ eyes were brown.
In total:
– 25% of children get BB (brown from both) – brown eyes
– 50% of children get Bb (brown from one parent blue from the other) – brown eyes
– 25% of children get bb (blue from both parents) – **blue eyes**
Even in this imagined 100% brown-eyed society, there is a 25% chance of any child having blue eyes.
Regressive *genes* do not diminish in prevalence each generation. It’s just that there are chances for them to not be expressed.
Imagine you have a blue eyed person married to a homozygous brown eyed person, and they have four kids, and we assume eyes are as simple as a single gene. bb x BB => Bb Bb Bb Bb. That’s four brown-eyed kids, but they aren’t homozygous like their brown-eyed parent. They are heterozygous and all carry the blue-eyed gene. 50% of all genes in this group are still blue-eyed genes. Nothing made blue-eyed genes disappear; they just got spread out so that they never get to take effect.
Imagine in the next generation, each of these carriers of blue-eyed-ness marries another carrier. Bb x Bb => BB Bb Bb bb. Half the genes are still blue! And now they are less spread out. One child of the two carriers is blue-eyed again, and two children are carriers.
Brown-eyed-ness overrides blue-eyed-ness in a single individual, but it is no more likely to be passed on to that individual’s children, so it never disappears, and it keeps coming back in later generations.
General: can we conclude that all blue eyes derive from a single person?
I guess that’s what I don’t understand.
Just because it’s not the dominant gene doesn’t mean it can’t be a common gene.
The gene for dwarfism is the dominant gene, but it’s not very common because it’s not beneficial to survival and two copies of that gene is fatal.
Polydactylism (extra fingers) is also a dominant gene, but the gene hasn’t been around long enough to be common in humans, but it has in cats, which is why polydactyl cats are a fairly common sight.
All that being the recessive gene means is that you need two copies of it to express it. Since blue eyes aren’t harmful in any way, once the gene for it entered the gene pool, it stuck around until eventually it was common enough that people could get two copies.
Blue eyes aren’t an actual color. They are caused by a lack of melanin in the iris. Most Northerners have adapted to have low amounts of melanin probably to absorb more Vitamin D in lower light conditions. The eye color is a side effect of this, but it’s why these genes are so prevalent. They naturally selected it.
You know the guy with peas? Recessive genes don’t disappear, they just vibe until they reappear like the Spanish inquisition
The answer is that it’s probably completely random. The theory is called [genetic drift](https://www.genome.gov/genetics-glossary/Genetic-Drift#:~:text=Genetic%20drift%20is%20the%20change,D.)
The effects of genetic drift are more pronounced in smaller populations. It’s likely that Northern Europeans are descended from a small population which ended up randomly all having blue eyes.
One of the tenets of genetic drift is that it ALWAYS leads to fixation of one of the alleles given enough time. We can observe this in nature and stimulate it in models, too.
Here’s more reading if you’d like to know more.
https://biology.sdsu.edu/pub/andy/Bohonak2008.pdf
https://www.nature.com/scitable/knowledge/library/natural-selection-genetic-drift-and-gene-flow-15186648/
https://pubmed.ncbi.nlm.nih.gov/11209765/
you cant use punnett squares to predict eye color, even though a lot of schools like to simply and teach it that way in high school it’s a huge oversimplification. It’s more complicated than just blue or brown. What about green? Hazel? There’s many kinds of eyes that are in between and a lot of different genes interact to make an iris look the way it does.
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