Understanding the Impact of Myopia: Heritability and Evolution in Human Eyesight

[t689]

According to some studies you'd have a 20% chance of an offspring being myopic with one progenator, while with 2 progenators, the chances are more like 50%. Then it says that there is a stronger sibling-sibling heridity than parent child. That is if one of your siblings has glasses, there is a stronger heritibility that you would have glasses versus if your parents have glasses. It seems to me that myopia is a disaster in the animal kingdom and even with humans, you'd much more likely to get lost or killed in the woods during man's days, and it would have been a disaster for you if you had myopia during those times, and even in some parts of the world today, you may just not find your way back to "home base" because you can't read the signs. So if you're a progenator and the children that don't have eyeglasses, does that mean that they have "bad genes" somewhere in them that will arise later farther along? Or is it like in some parts of the world where myopia is an epidemic, something that was caused by many progenators, and the weight of the myopia just kept building, and it will take an equal weight of natural selection of many generations to clear the myopia out of those areas? Kind of like ridding an area of the plague or a deadly disease.

 

[DaveC426913]

It seems to me that myopia is a disaster in the animal kingdom and even with humans, you'd much more likely to get lost or killed in the woods during man's days, and it would have been a disaster for you if you had myopia during those times,

Genes rarely work in a vacuum.

<speculation>
Myopia might be associated with some benefical genetic trait that offsets the disadvantage of poor eyesight.
For example, it could be closely tied to, say, cranial cavity size.
</speculation>

If the gene pool had to sacrifice some good vision for say a big brain, it would average out as an advantage.

 

[Ygggdrasil]

there is a stronger sibling-sibling heridity than parent child. That is if one of your siblings has glasses, there is a stronger heritibility that you would have glasses versus if your parents have glasses.

Because siblings are more likely to be raised in similar environments than parents/children, this fact says that there is some environmental component to developing myopia (which is true). For example, I believe I've read that time spent indoors vs outdoors correlates with the risk of developing myopia.

 

[t689]

And a recent more comprehensive analysis of the data realized it was all genetic as is obvious. The eyeball, the cavity, and the cornea are all shaped and grown by genetic factors. They even cut the optic nerve of newborn chicks, and those reared to be refractive, even after cutting with no light input, the eyeball grew to a longer more refractive length as it was reared to be refractive. But it does say 1 in 5 are the general odds if one parent is myopic, so it must mean that the influence from the adult that has no refractive errors outweighs to some extent those of the progenator. So there is some wiggle room apparently with a myopia progenator.

 

[Fervent Freyja]

I wouldn't say it was additive through genetic inheritance, but because of adding certain conditions that may cause myopia.

Like ygggdrasil noted, we are no longer in an environment where we constantly use our eyes to see far distances. Much of our vision is controlled by muscles and can be trained, but some aspects are set in development (where children are using more near vision than ever before). Little exercise of the muscle movements involved in seeing far distances will result in degradation. Overuse can result in myopia.

"The eye can focus objects at different distances because the ciliary muscles push and pull to make the lens change shape."

 

[Drakkith]

And a recent more comprehensive analysis of the data realized it was all genetic as is obvious.

Which analysis? Please provide a link to this reference if possible.

 

[t689]

It's genetic. To deny otherwise is delusion. But to point out the first responder, apparently you have to choose if there are benefits that otherwise would outweigh what might be a slight blemish on the genetic record.

 

[256bits]

It's genetic. To deny otherwise is delusion.

Are you denying that environment has no effect whatsoever?

 

[t689]

I think I changed my mind. I think that myopia poisons the gene pool.

 

[Ygggdrasil]

From the American Optometric Association:

Even though the tendency to develop myopia may be inherited, its actual development may be affected by how a person uses his or her eyes. Individuals who spend considerable time reading, working at a computer, or doing other intense close visual work may be more likely to develop myopia.

http://www.aoa.org/patients-and-pub...ary-of-eye-and-vision-conditions/myopia?sso=y

Perhaps the genes that predispose myopia were not deleterious in human ancestors because they did not live in environments or engage in activities that would lead to the development of myopia.

Here's a citation to the study I mentioned in my previous post linking time spent outdoors with a decreased risk of myopia, further demonstrating a non-genetic component to the cause of myopia:
http://jama.jamanetwork.com/article.aspx?articleid=2441261

 

[jim mcnamara]

@t689 -
First:
In Freshman Biology one of the first things you learn is the heredity-environment interaction Example:
Identical twins start with identical DNA, at age 40 their DNA behaves differently - because of chemical changes that activate/deactivate some genetic sequences.
This kind of study is called Epigenetics. And it helps to explain why people with the same DNA get different diseases and so on. Because their environment was different: diet, exercise, long term exposure to tobacco smoke, etc. This helps explain the observation in the link above.

Next:
You seem to think that "inferior" genes will be selected against. This seems to imply you think myopia is a recessive gene. AFAIK there are no studies to show that a single gene influences myopia. But let's assume there is one. Let's look at Sickle Cell Anemia.
One of the things you learn in genetics is that recessive and clearly detrimental genes
[ that produce diseases that can kill off persons who are homozygous for the gene ]
are actually beneficial. How? A person living in a malaria infested environment does much better in surviving malaria when they have one sickle cell gene and one "normal" gene. They are heterozygous for the gene. They are at a huge survival advantage compared to persons who have either all Sickle cells genes or none.

It doesn't appear to me that you read any of the links given. However, if you want some Genetics links we can provide them

PS: please post references in the future. PF does not support posts based on scientifically unsupported concepts.

 

[t689]

Here's a good study: http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1003299

It's a study on 45,000 caucasian people genetically. The results were that they identified genes that are implicated in the eventual need for glasses. Those children that had the "bad genes" in the 90th percentile (the worst) risk group (survival probability of surviving without need for any glasses) had a probability of ending out with no need for glasses of 40%. So what they're saying is that they identified genes or "risk alleles" that if you end up with those, you have a certain probability of NOT needing glasses (their survival probability if you carry those genes). Apparantly you can end up with risk alleleles and still not develop myopia. Also you can carry those genes and develop myopia. Also there is no analysis of two adults one with myopia or two with myopia. I think what they're saying is that if you inherit the bad genes you have a certain probability of ending out with no need for glasses. But the study outlines what they're saying and they do mention the fact that there has been an environmental hypothesis, but what they're looking at is risk alleleles on genes and what they seemed to have concluded is that whether you end up with glasses is based on what you inherited but also the survival probability among the entire group that all ended up with "risk alleles" with in the worst case a 40% chance of not needing glasses, and the process is linked to the very early growth of the brain, in particular the retinal cells. But to be analytical, they discovered that even among those who inherit the risk genes that they looked at in their study, you have a probability at worst case of 40% surviving without the need for glasses. And to repeat 60% of those who inherited the "bad genes" at worst case ended up with a need for glasses. And to add another loophole, they do no analysis on if a person with glasses mates with a person with no glasses, they only looked at the genes of the group and analyses. So in theory it may be likely that one person with glasses mating one person with not glasses the chances of a child not needing glasses is 80% among a random group of people, but probably amongst the whole more like 70% if one of them has glasses. ---So my take on this is that everybody inherits or ends up with two sets of genes, one selected from the mother and one selected from the father. The ones that end up with no need for glasses but perhaps had a myopic parent inherited "bad genes" but they ended up not being expressed perhaps recessive I'm not sure. Or on the flip side the ones that were expressed were the ones that did not need any need for glasses, but perhaps when THEY MATE [[my take on this is that those who have the risk alleleles and ended up with no need for glasses have the risk gene amongst their genes but the one that is expressed on that site came from the other parent who passed down a gene that was not a risk allele on that site]] are identified they still carry the risk for carrying on genes to a child that will perhaps statistically add a very slight increase in the need for glasses. Many large famillies have and still do not have any members going back 100 years with no need for glasses. Other areas, perhaps in asian cities, over 3 generations have built up a "weight of refractive propensity" that is expressed above 80% in that city. So the way I would look at it is where you are located and if not that many people have glasses and you have glasses, it doesn't seem to be any problem with mating with a large number of them and not really affecting future generations, because who knows in the future eyeglasses may be outlawed as a deterrent to natural selection. Labrador retrievers in some areas who keep over 100 of them, 30% end up with refractive errors, but they mate constantly, and there are progenators who are myopic, but it must balance out amongst the nattural selection on those estates to where only 30% need glasses, and it doesn't overwhelm the group even though you may have a progenator in one generation who is dominant and mates with all the female dogs, the probability is that the next generation the progenator is still one who is free of refractive errors, dominant, and mates with all the other dogs. And to further add a note in some studies myopia with greater than refraction -6 diopters has been discovered to be autosomal dominant. If say your parent was not high myopic but needed glasses and your other parent had no person in a large family going back generations with any need for glasses, then even though you have glasses the odds are that identifying those risk alleles that you do carry from the progenator, when YOU MATE the ones passed down to the offspring are statstically speaking going to de-select those "bad genes" since the one parent had no risk for myopia, but statistically, you in what you are looking through ended up with an eight ball, but the sheer probability is that the child will not need glasses especially if the one you mate does not need glasses since you have no autosomal dominant genes, you're just as likely to pass down the "good gene" from the non-progenator in your parents. So let's say that of 20 genes that contribute to myopia, you inherit ten of them and you cannot have more than 6 to develop myopia. And let's say that ideally two people without glasses the child has a 1 in 40 chance of needing glasses. To get 1 in 40 with two parents with no need for glasses, that would mean that one of the parents (not both) has a bad gene. So let's assume your partner has 2 bad gene out of the 40 genes they care considering they have 2 copies of genese one from each parent., and you have 10 bad genes out of 20 or really 20 out of 40. When these combine, just roughly speaking, you can simplify it to 10 out of 20 combining with 2 out of 20. Those are 400 possible gene pairs, with at worst case a stack of 220 bad gene pairs. 220 divided by 400 is 0.55. And at the worst case 0.55 times 10 is 5.5 which does not equal the six which you had said is necessary to have glasses.

 

[jim mcnamara]

Good. You got the idea: genes are not destiny, and it can be complex to determine outcomes based only on certain genes implicated in the development/non-development of myopia.

 

[Fervent Freyja]

I think I changed my mind. I think that myopia poisons the gene pool.

Wait. What is your aim here? Are you seriously trying to build up a case to show that Caucasians are poisoning the gene pool? Get real now...

 

[tu anciana abuela]

My high myopia -since I was a child- has as a consequence my acute short sight seing even now, at my old age (almost 70).

In the paleolitic/old times, my good short sight would have enabIed me -more than other members of the tribe, to clean and cure wounds, manufacture and manipulate small tools, sew up bison coats for young and old ones ...

Life is/was not only running up and down looking/avoiding for/being prays/a pray. Sitting by the fire, chatting and doing handwork was also an important part of it.

(beg you excuse my poor language)

;)

 

[DaveC426913]

I think I changed my mind. I think that myopia poisons the gene pool.

Yup. Some of us are poisoned with near-sightedness and giant brains.

 

[jim mcnamara]

I'm with su anciana abuela (my elder grandmother, too). There are a some hypothesis about multigenerational families and clans and how it increases the efficiency of the group. See E O Wilson: The Social Conquest of Earth

I'm past 70. Got lucky and an operation restored my 20/200 vision to normal. I'd rather be lucky than good.