Why do people have blue eyes?

[Genghis Pwn]

Most of the world's population have brown irises. Why do some white people have blue and green eyes?

[arcticpenguin]

The physical "how" is that they lack a gene for producing the brown pigment.

I think "why" is still controversial. Some people have claimed that it is better for seeing in dim light at higher latitudes. Others claim it is sexual selection (i.e. potential mates dig it).

Did I leave out any other proposed explanations?

Since it is a historical question, not an experimental one, we may never have a definitive answer. I myself would probably lean towards the sexual selection explanation.

[Genghis Pwn]

penguin, I suspected that. Blue eyes are attractive! If it were latitude, why wouldn't people in northern china have blue or green eyes, for instance?

[Diamond]

Originally posted by Genghis Pwn
penguin, I suspected that. Blue eyes are attractive! If it were latitude, why wouldn't people in northern china have blue or green eyes, for instance?

Quite a few Mongols, I believe, have green eyes.

[Leif Roar]

Originally posted by Genghis Pwn
penguin, I suspected that. Blue eyes are attractive! If it were latitude, why wouldn't people in northern china have blue or green eyes, for instance?

Because the people in Northern China might not be from a population where that particular mutation had occured. Evolution isn't determenistic, so even though a particular trait might be advantageous, it's not certain that this trait will develope in a population.

That said, I neither support nor oppose the idea that blue and green eyes developed for seeing in dim light at higher lattitudes - I don't know near enough about the reasoning behind that suggestion to even make an assumption about it.

[asthmatic camel]

My mother has one green eye and one blue, explain that !

[Genghis Pwn]

Originally posted by Diamond


Quite a few Mongols, I believe, have green eyes.

That could be due to their Turkish ancestoral link, or the Aryan migrations, which left people with green eyes in place like northern india, Afghansitan, and Iran. Also, I don't think many mongols have green eyes.

[arcticpenguin]

Originally posted by asthmatic camel
My mother has one green eye and one blue, explain that !
I've seen several Siberian Huskies with one brown eye and one blue.

Is your mother a dog? :p

[AmateurScientist]

Indeed many "black" persons with one white parent and one black parent have blues eyes. The contrast can be quite striking and beautiful.

As AP suggested, it's probably sexual selection. They doesn't seem to be any survival benefit to blue eyes, although I have heard that perhaps blue-eyed persons tend to have slightly better depth perception. As I don't understand how the lack of pigment would contribute to that, I don't really have any idea if it's true or not.

AS

[arcticpenguin]

Originally posted by AmateurScientist
Indeed many "black" persons with one white parent and one black parent have blues eyes. The contrast can be quite striking and beautiful.

I think the brown-eyed genes are dominant over the bue-eyed genes, so this is possible but may not be all that common. Think Tyra Banks.

[AmateurScientist]

Originally posted by arcticpenguin

I think the brown-eyed genes are dominant over the bue-eyed genes, so this is possible but may not be all that common. Think Tyra Banks.

And yet those recessive genes sometimes take hold, don't they?

Anyway, I've seen several predominately "black" persons in my city personally who have blue eyes. It certainly isn't the norm, but plenty of such folks exist.

AS

[Yahweh]

My eyes are green. My wife's eyes are very very green. I'll go with arcticpenguin's sexual selection explanation.

[asthmatic camel]

Originally posted by arcticpenguin

I've seen several Siberian Huskies with one brown eye and one blue.

Is your mother a dog? :p

Of course she isn't a dog, she's a camel, fool !

[BillyJoe]

If blue eyes are due to a lack of brown pigment, what causes green eyes?
Is it a bit of brown pigment?

The reason I ask is that I have green eyes but most people will say that my eyes are brown until I ask them to take a closer look.

(Not a bad ploy, by the way, it that person is female! )

[Filippo Lippi]

Originally posted by asthmatic camel
My mother has one green eye and one blue, explain that !

You mother is David Bowie?

[Genghis Pwn]

Originally posted by BillyJoe
If blue eyes are due to a lack of brown pigment, what causes green eyes?
Is it a bit of brown pigment?

The reason I ask is that I have green eyes but most people will say that my eyes are brown until I ask them to take a closer look.

(Not a bad ploy, by the way, it that person is female! )

Good question, BillyJoe. Mine are green, but they have a brown-hazel "star" in the middle, radiating from the puplil's edge outward into the green iris.

[Badger]

Originally posted by asthmatic camel


Of course she isn't a dog, she's a camel, fool !

A Siberian camel?;)

[quote]That could be due to their Turkish ancestoral link, or the Aryan migrations, which left people with green eyes in place like northern india, Afghansitan, and Iran. Also, I don't think many mongols have green eyes.{/quote]

You have that backwards. The Mongols traveled west, and left their genes behind in all the places you mention.

[arcticpenguin]

There's an article on sexual selection in the July 2003 National Geographic. They don't say anything about humans or eye color, but they have some nice pictures of other species.

[Genghis Pwn]

Originally posted by Badger


A Siberian camel?;)

[quote]That could be due to their Turkish ancestoral link, or the Aryan migrations, which left people with green eyes in place like northern india, Afghansitan, and Iran. Also, I don't think many mongols have green eyes.{/quote]

You have that backwards. The Mongols traveled west, and left their genes behind in all the places you mention.

Um, no, the traveling went both ways. Before the Great Khan's westward invasions, Turkic peoples were traveling and mixing east across the steppes of Asia. Also, like I said, Aryans with green and blue eyes were spreading across asia, leaving populations in Northern India, Afghanistan, etc. The Greeks had also been all the way into the Hindu Kush with Alexander, and left blue-eyed peoples there. Indeed, the Greeks even mixed with Buddhists there to produce an amazing hybrid Greek-Buddhist culture.

It is also believed there may have been pockets of proto-Aryan peoples who survived in the high mountains of Afghanstian all along, as most Aryans had gone west and were busy populating Europe.

Anyway, it's possible that the Turks, Mongols, Greeks and Aryans crossed paths enough times and mixed it up enough that a few Mongols like the Great Universal Khan himself (Genghis!) could have had green eyes, but not from Mongolian genes.

[Badger]

Originally posted by Genghis Pwn


Um, no, the traveling went both ways. Before the Great Khan's westward invasions, Turkic peoples were traveling and mixing east across the steppes of Asia. Also, like I said, Aryans with green and blue eyes were spreading across asia, leaving populations in Northern India, Afghanistan, etc. The Greeks had also been all the way into the Hindu Kush with Alexander, and left blue-eyed peoples there. Indeed, the Greeks even mixed with Buddhists there to produce an amazing hybrid Greek-Buddhist culture.

It is also believed there may have been pockets of proto-Aryan peoples who survived in the high mountains of Afghanstian all along, as most Aryans had gone west and were busy populating Europe.

Anyway, it's possible that the Turks, Mongols, Greeks and Aryans crossed paths enough times and mixed it up enough that a few Mongols like the Great Universal Khan himself (Genghis!) could have had green eyes, but not from Mongolian genes.

I stand corrected. The area is indeed a crossroads for many peoples, going in many directions.

[athon]

Originally posted by BillyJoe
If blue eyes are due to a lack of brown pigment, what causes green eyes?
Is it a bit of brown pigment?

The reason I ask is that I have green eyes but most people will say that my eyes are brown until I ask them to take a closer look.

(Not a bad ploy, by the way, it that person is female! )

The iris isn't a single layer of muscle. There are about ten layers of tissue that have a 'stringy' nature, allowing lower layers to show through. For some reason, pigment can be made in lower layers, and not the closest. Unlike what they tell you in high school science, there are about 8 genes coding for eye colour. Several of these code for positioning of melanin in the iris, accounting for variation in colour and patterns.

As for different eye colours in one individual, this is 'mosaic' type effect, much like tortoise-shell patterns in cats. Both eyes have all of the genes for the colour - one eye, for some reason, doesn't switch on all of them, being then green or blue.

Athon

[BillyJoe]

Athon,

Do you mean that green eyes is the result of brown pigment in the deeper layers of the iris overlain by blue (= lack of brown pigment) in the superficial layers?

Also regarding mosaicism....
Isn't this only seen in females - I seem to remember this form Hellcat's thread. If so how come David Bowie has one brown and one blue eye. Is there something he is not telling us?

BillyJoe.
(Both same colour)

[davidhorman]

If so how come David Bowie has one brown and one blue eye. Is there something he is not telling us?

This is without confirmation (due to laziness on my part) but Bowie's eyes aren't different colours. One looks darker because it's pupil is permanently dilated due to a head injury.

David

[arcticpenguin]

Originally posted by BillyJoe
Athon,
Also regarding mosaicism....
Isn't this only seen in females - I seem to remember this form Hellcat's thread.
Yes "Barr bodies" are only seen in females, but other types of mosaicism can happen in either sex. Think of the example already given - tortoise shell cats. The genes are there, but the signals for turning them on somehow were not the same during development.

Other types of mosaicism exist. Especially in plants you can have a transposon (jumping gene) actually cause a mutation in different parts of the body during growth. Classic example: 'Indian' corn.

[arcticpenguin]

Originally posted by davidhorman

This is without confirmation (due to laziness on my part) but Bowie's eyes aren't different colours. One looks darker because it's pupil is permanently dilated due to a head injury.
David
http://www.bbc.co.uk/dna/h2g2/alabaster/A646995

At school he acquired his famous differently coloured eyes in a fight when a boy stuck a compass in his eye. As a result one of his pupils is permanently enlarged.

[jayrev]

My right eye is blue/green. My left eye is mostly blue/green with about 1/3 wedge of the iris being brown. I've never seen any one else with eyes like mine. Any ideas why?

[popsy]

I don't think that it's necessary that there be a reason for blue/green eyes. If the lack of brown pigment didn't cause a loss of survival fitness, it could continue...or not. Apparently, it continued. Yes, it's a recessive trait, but if it were appropriately widespread, it would still pop up. My sister had very dark brown eyes. Her children have blue eyes, so she must have carried the recessive gene. My mother had gray eyes, but all of her children had brown eyes. My uncle and his wife both have blue/gray eyes, their daughter has brown. A person can have brown eyes that appear blue or gray, due to a brown-inhibitor. The brown gene is there but not expressed.

My opinion is that there is so much variability in eye color because it probably doesn't matter much as long as there is some pigment present for protection. Not all cultures find a particular color attractive.

[ImpyTimpy]

Ok, I have a question, can someone tell me why a person might have pitch black eyes? I've only once seen a woman with pitch black eyes (not blue, not green, not brown - black!) and found it to be a very interesting. Any ideas as to why she'd have pitch black eyes?

[calladus]

Originally posted by arcticpenguin

I've seen several Siberian Huskies with one brown eye and one blue.

Is your mother a dog? :p

A Were-Huskie?

[UnrepentantSinner]

Originally posted by Genghis Pwn


That could be due to their Turkish ancestoral link, or the Aryan migrations, which left people with green eyes in place like northern india, Afghansitan, and Iran. Also, I don't think many mongols have green eyes.

I wouldn't even know where to being pointing out the errors in this statement.

[Badger]

Originally posted by UnrepentantSinner


I wouldn't even know where to being pointing out the errors in this statement.

Could you please begin somewhere? I'd like to know the fallacies in the statement.

Thanks.

[Hominoidea]

Originally posted by ImpyTimpy
Ok, I have a question, can someone tell me why a person might have pitch black eyes? I've only once seen a woman with pitch black eyes (not blue, not green, not brown - black!) and found it to be a very interesting. Any ideas as to why she'd have pitch black eyes?

She had contacts. Those colored types.

[popsy]

Originally posted by ImpyTimpy
Ok, I have a question, can someone tell me why a person might have pitch black eyes? I've only once seen a woman with pitch black eyes (not blue, not green, not brown - black!) and found it to be a very interesting. Any ideas as to why she'd have pitch black eyes?

Brown pigment can appear black if it doesn't reflect enough light to show color. There are many degrees of brown pigment possible. My father's eyes were almost yellow, but his brother's were near-black. Both had 'brown' eyes.

[UnrepentantSinner]

Originally posted by Badger


Could you please begin somewhere? I'd like to know the fallacies in the statement.

Thanks.

Sorry for not replying sooner, but I've been trying to distill a long rambling reply down to a short rambling reply. :)

To preface, light hair and eye color is found nearly exclusively in the Celts, Germans and Slavs of Northern Europe. It's the result of an environmental selection (less sunlight means less need for dark hair and eyes) resulting over generations of population isolation in traits that eventually became primarily sexual selectors. Why is the population isolation important? Because light hair and eye color are recessive traits.

This population isolation extends back at least to the Neolithic period.

Now about Porn's comment:

"That could be due to their Turkish ancestoral link, or the Aryan migrations, which left people with green eyes in place like northern india, Afghansitan, and Iran. Also, I don't think many mongols have green eyes."

He's right about not many Mongols having green eyes, at least from what I have seen. Apart from that...

- The Turks and Mongols are two distinct groups. The Mongols coming from what is now Mongolia and the steppes of Northwestern China and Southern Siberia. The Turks came from futher east, mainly from what was called Soviet Central Asia.

- Neither group would have anything to do with the Aryan migrations that supposedly occured from Iran and the Caucasues into India and Europe, especially since the Aryan migrations supposedly occured during the Neolithic period and they weren't distinct cultural groups until much later. It's a lot like saying the Toltecs and Mayans were active during the Clovis Period.

- The Aryan migrations is mostly theorectical and based on rather flimsy 19th Century Archaeology. There just isn't a whole lot of evidence for the "Out of Iran" theory for the language and population characteristics of everyone from Gudrathi's to Celts.

- Where we do likely have infusions of light hair and eye color genes is from the reverse theory which is Copper and Bronze Age traders or hunting parties from Europe making their way into the east (see European featured Chinese mummies) and from the children fathered by Greek soldiers following Alexander's conquest of south central Asia.

[RSLancastr]

Originally posted by asthmatic camel
My mother has one green eye and one blue, explain that ! Her ancestors lived on the equator, and always faced East.

[RSLancastr]

Originally posted by AmateurScientist
Anyway, I've seen several predominately "black" persons in my city personally who have blue eyes. It certainly isn't the norm, but plenty of such folks exist.I am sure that such persons exist, but with today's cosmetic contact lenses, who knows?

[RSLancastr]

Originally posted by Badger
I stand corrected. The area is indeed a crossroads for many peoples, going in many directions. Sorta like San Francisco?

[Badger]

Thanks for the explanation, Unrepentantsinner! I appreciate it.

[ImpyTimpy]

But doesn't that mean that the pigment is in fact black or in your case - yellow in colour? Colours afterall are created when certain frequencies of light are absorbed while others reflect back.

Originally posted by popsy


Brown pigment can appear black if it doesn't reflect enough light to show color. There are many degrees of brown pigment possible. My father's eyes were almost yellow, but his brother's were near-black. Both had 'brown' eyes.

[popsy]

Originally posted by ImpyTimpy
But doesn't that mean that the pigment is in fact black or in your case - yellow in colour? Colours afterall are created when certain frequencies of light are absorbed while others reflect back.



Human pigment comes in only 2 colors. Eumelanin produces brown/black, and pheomelanin produces red/amber. The amount of pigment present determines the color that we see. Lots of eumelanin will cause black appearing eyes, very little of it will cause the eyes to appear very pale, yellowish brown. Even less pigment present will cause the eyes to be blue. Here's a pretty good explanation of eye-color genetics;

http://www.seps.org/cvoracle/faq/eyecolor.html