After hearing of an article in Harpers or The New Yorker (?)about the “Disappearing Blond(e),” I got to wondering if it can be statistically established how quickly the world’s population of blue-eyed people are becoming less prevalent.
Obviously, the birth rate in the “Third World” greatly outpaces that of the western world–and considering that the populations therein are overwhelmingly brown eyed–what inferences as to relative blue-eyed populations, can one mathematically estimate with some high degree of confidence when the last generation of blue-eyed people will live on Earth?
Considering that the majority of blonde-haired, blue-eyed people live in first world countries, I supposed that they will remain relatively isolated and the traits could be maintained indefinitely.
I suppose it would be just as reasonable to ask, since most of the world is not as dark-skinned as, say, the Maasai, when the last generation of truly black people will exist.
Your question supposes that people will mix more or less at random and evenly in the future. While the number of people of mixed heritage will no doubt increase, this does not mean the more extreme ethnic types will go extinct. There is a certain number of people marrying within the group as well as folks marrying out.
Also, just because a blue-eyed person marries a brown eyed person doesn’t mean the gene goes extinct, either. Quite a few people with brown-eye genes carry genes for blue eyes as well, and when they marry - however brown-eyed those two people may be - they can produce blue-eyed children.
In other words, if a Swede marries a Zulu their children will be dark-eyed and dark skinned, but if those children then marry people with traits more like Swedes than Zulus then the grandchildren of the original mix may, in fact, be relatively light skinned and even, yes, blue eyed. This is why some descendants of Africans enslaved in this country were able to “pass” as white - because certain genes lined up and the result was a person that took after their European ancestors more than their African ancestors.
So even if during one generation all the blue eyed genes are swamped by the brown-eyed genes, the next generation (and the next, and the next…) still retains the capacity to produce blue eyes.
I don’t know where exactly I was going with the “blonde-haired, blue-eyed people living in first world countries” bit. They will probably have a greater chance of survival because of access to better medical attention and a generally higher quality of nutrition, maybe.
Let’s put it this way, by the time the Aryans are even close to extinct, blonde hair and blue eyed children will probably be mass produced in test tubes and gestated in the wombs of kangaroos.
I was taught in school that the gene for blue eyes is recessive.
Of four offspring from blue/brown couplings, statistically, 3 would have brown eyes and only one blue … But the blue-eyed child would carry the dominant brown eye gene.
Actually, if blue is recessive then the blue-eyed kid will be the only one NOT carrying a brown-eyed gene. Your statistic would only be accurate if the brown-eyed parent was blue/brown heterozygous. If the parent was homozygous brown then all of their offspring will be brown-eyed; no chance at all of blue eyed kids. This is assuming of course, that the paradigm is as simple as brown=dominant, blue=recessive and that there are only 2 colours available. There is actually much more to it than that though.
Many years ago I read an article about someone who had done hard statistical look at the question. As I recall there was some sort of surprise conclusion - something like even once the genes were randomly mixed worldwide, blue-eyed people would make up 50% of the population. (It’s been a loooong time, so don’t quote me.) Anyone care to present a statistical analysis to this question?
The gene for brown eyes is dominant (B). The gene for blue eyes is recessive (b). You get one gene from each parent. A phenotypically brown-eyed person could have one of two genotypes–either BB (one brown-eyed gene from each parent) or Bb (a brown-eyed gene from one parent and a blue-eyed gene from another). A phenotypically blue-eyed person can only have one genotype–bb (one blue-eyed gene from each parent.)
Therefore, a blue-eyed person cannot carry the brown-eyed gene, but a brown-eyed person can carry the blue-eyed gene. If two brown-eyed people who each carry the recessive blue-eyed gene have children, they have a 1-in-4 chance of having a blue-eyed child as you described above. That child would not carry any brown-eyed genes, though. They have a 1-in-2 chance of having a brown-eyed child who carries a blue gene, and a 1-in-4 chance of having a brown-eyed child who does not carry the blue gene.
(Don’t ask me how we get green and hazel-eyed people. We never really seemed to get to that, even at the introductory college level. Therefore, I doubt that it is always this simple–probably actually more than one gene involved.)
Since there are so many carriers of the blue-eyed trait, I doubt that it will ever truly die out in the human population. It may just “pop up” surprisingly. Heck, my husband and I are both dark-haired, and we have a blond child. Sometimes, hidden recessives do get together. We’re lucky when it is with innocuous results.
Thanx for the correction, and the memory jog. I remember the ‘big B’ ‘little b’ lesson now. … and I didn’t mean to tell a lie, and I did hand my homework in … and it was very many years ago!
Cheers
Assuming that the statistic is true, they probably meant that 50% of the population would carry the allele for blue eyes, not that 50% would express it. Since blue eyes are recessive, many more people carry the trait than are blue eyed. To get the frequency of the trait, simply take the square-root of the rate of expression (assuming random mating). So, if .25 of the population has blue eyes, then the blue allele is at .5 frequency.
However, I don’t think the frequency of the blue allele could be that high worldwide. In Africa and Asia the frequency is close to zero.
And so what inference could be made about the prevalence of blue-eyed people in the United States over the coming decades or centuries. Given that the gene for brown eyes is dominant and blue eyes recessive–plus factoring in the accelerating intermarriage between ethnic groups, what assumptions can be made re: the OP? Can we therefore conclude that blue-eyed people will become increasingly rare?
Ok - heres a quick look at the math - straight out of my genetics notes, so if there are any errors, its probably why i didn’t ace the exams, and besides, Im tired.
This equation can be used to determine if detrimental genes can ever be completely eliminated. The blue eyed gene is not detrimental, but lets assume that everyone who carried it died or didn’t reproduce.
Using frequency of recessive gene equation :
qn = qo / (1 + nqo),
where qo = initial recessive gene frequency
n = number of generations
qn = gene frequency after n generations
Assume an initial gene frequency of 0.25. Using the above equation, after 1000 years, the frequency would be reduced to 0.001. It would take a VERY long time to deliberately eliminate the blue-eyed gene.
Hence, it cannot be eliminated totally even when it is INTENDED to remove the gene from the human population. This was a major flaw in the German Nazi regime’s thinking.
The OP was asking about random elimination of the blue-eyed gene - I think we don’t have to worry.
As for hazel or green eyes - didn’t Cecil answer this? It’s fat, silly (and hence not determined by the same single gene)
I don’t believe that eye color is quite as simple as a two allele Punnet square. IIRC there is a lot more room for variance in eye color although blue eyes are recessive.
Blue or Brown is controlled by a single gene - two alleles. Variation in darkness/lighness/greenishness/amberishness is due to different factors, such as lipids in the irises. These factors are likely to have multiple genes, IIRC.
Actually, this site has an explanation of how green eyes can be explained by taking a second known gene into account. According to this page other eye colors, such as hazel and gray, still aren’t understood.
I’ve always liked Mendel. My eyes are techinally blue since both my parents are blue eyed (bb + bb) but some times they’re gray, some times they’re green, and some times they’re blue green(blue green =irritated eyes/lack of sleep. Not worth the lovely color) Usually they’re blue, though, so I’ve only had one argument so far over if they’re blue or not Funny, though, until that argument, I never realized they weren’t always blue!
I don’t think any high degree of confidence can be assigned to the query from the OP. Although I think it is likely that there will always be blue-eyed humans.
In order to eliminate blue eyes, the world would have to reach a point where those who carried the b allele did not tend to mate amongst themselves. At this point in time, mating would be independent of eye color traits and the differing first/third world birth rates would no longer matter.
At this point in time would the number of people with blue eyes decrease? Yes
Would they ever be eliminated? Likely not
The recessive blue-eye allele represents some percentage of all alleles. Since each allele has a 50-50 chance of being passed on to the next generation, the ratio of B to b should remain fairly constant. The percentage of the population that has blue eyes would be approximately the square of the percentage of b alleles.
This is not to say that blue eyes could not be eliminated eventually. There is some chance that it could occur. But there is also some chance that everyone in the world might end up with blue eyes.
