Is it known whether poor eyesight (both nearsightedness and farsightedness) is a dominant or recessive trait; or is it yet unknown or a combination of different genes?
-An undergraduate student from Canada
The genetics of poor eyesight are too complicated to be a simple dominant or recessive trait. Something like "nearsightedness" is probably made up of more than one disease and each disease probably has more than one gene involved!
Poor eyesight definitely runs in families. Recent studies have shown that if both your parents are nearsighted, then you have about a 1 in 3 chance of being nearsighted too. If only 1 of your parents is nearsighted, then you have a 1 in 5 chance of being nearsighted. If neither of your parents is nearsighted, then you have less than a 1 in 40 chance.
So is it genetic? Or do families that read a lot or watch too much TV have a higher chance of having poor eyesight? In other words, are genes, the environment, or a combination of the two involved?
The study where the numbers I report came from is no help. They find that nearsighted kids spend around 2 more hours/week reading and studying than do kids with normal vision. Again, are nearsighted kids drawn more to these activities or did these activities cause the nearsightedness?
To try to figure out what role the environment plays, scientists often do a twin study. In a twin study, identical twins are compared to fraternal twins. If something happens more often in identical twins, then it probably runs in the family.
How does a twin study show something runs in a family? Remember, identical twins have exactly the same genes. Fraternal twins share only as many genes as any brother or sister.
Because twins are born at the same time, the environment is as same as possible for them. So if something happens more often in identical than in fraternal twins, then it is most likely because they share the same genes.
A recent twin study suggested that around 90% of all near and farsightedness involved genes. Of course this doesn't mean that the environment doesn't play a role. It may be that many of these genes only increase a person's chances of having bad eyesight. For the genes to take effect, something in the environment has to trigger them.
For example, 30 years ago, around 25% of the teenagers in Singapore were nearsighted. That number is now around 80%. The difference isn't a change in the gene pool; it is more what the kids are doing these days.
They are spending more time studying, watching TV and playing video games and less time playing outside. Reading, TV, and video games seem to increase your chances of becoming nearsighted because the eye changes shape to focus more clearly on things up close. Sports, on the other hand, usually make you focus on more distant objects so that keeps the eye from changing shape. Or at least that is how the theory goes.
Nearsightedness happens when the eye is elongated -- the eye can no longer focus well on faraway objects. Genes might be involved in two ways.
In the first, the genes tell your eye to come out elongated. These people will have bad eyesight no matter what they do.
The second way is that the genes somehow make the eye more malleable -- it changes more easily. These genes would make the person more susceptible to the effects of watching TV and reading. If they spend some time playing outside too, then they might escape nearsightedness.
As you can see, this is a really complex subject! Lots of genes, more than one disease and things we do all mix together to determine our eyesight.