Nature v nurture: It’s a question which affects so many developmental debates as paediatricians, physicians, and others seek to understand the physical and mental changes which an individual may encounter on their road to adulthood. In some instances, the answer may be obvious; but in others, the debate rolls on, with theories being formed and reformed as our knowledge of genetics and the environment grows.
Take myopia, for example. The tendency for myopia to run in families may, at first sight, suggest that genetics plays a part in its development. But could that link be less nature and more nurture? It has been shown that close work can be a trigger for myopia, so are families with myopia more likely to be book readers or to spend a lot of time on computers?
One of the ways in which researchers might test a link is to undertake a review of twins. If both twins have myopia, does that strengthen the argument for a genetic link? Well, it might do, but here again, we are back with the book/computer argument. And if only one twin has marked myopia, does that argue against an inherited condition? Well, it might do, but one twin may be bookish and one sporty, and it is known that time spent outdoors can help to reduce the development of myopia.
This doesn’t mean that research cannot identify whether myopia is caused by inherited genetic or by environmental factors. But it does mean that researchers have to be rigorous in the way in which they select and manage population data. And when they do so, myopia emerges as a complex condition which can be caused by a mix of genetic and environmental factors.
Parental myopia is now generally recognised as playing its part, with one study [1] commenting that:
“parental myopia has a significant positive association with a child's risk of developing myopia;” adding, “Children of two parents with myopia have a higher risk of developing myopia than those who have one parent with myopia.”
With that in mind, what genetic factors might play their part in increasing the risk of developing myopia? The first breakthrough came in 1981 [2] when a genetic locus (the fixed point on a chromosome where a genetic marker is found) MYP1 was identified in a Greek family. This particular marker was interesting in that it only affected the male line. Since that time, further markers have been identified; with one 2020 study alone identifying 336 novel genetic loci associated with refractive errors, including those caused by myopia.[3] And scientists believe that that is only the start, with more investigation required in order to fully map the effects of genetic instructions within the eye.
Why so many? Well, for a start, the eye is incredibly complex, so different genes can influence the chance of developing myopia in different ways. Let’s start by looking at one of the more common aspects of myopia, the abnormal growth of the eyeball which leads to light being focused in the wrong place within the eye. If an inherited gene variant contains the instruction that eye length is greater than normal, then myopia will follow. Similarly, if a gene code makes the eye continue to grow beyond normal limits, then the chance of developing myopia is greater than normal.
There are also other aspects of the eye which can influence the development of myopia. How the eye develops is partially dependent on the amount of light which it receives. That’s why spending time outdoors is an important myopia management tool.
But we also know that eye colour can make a difference to light management within the eye and that light management is associated with eye growth. So one study which revealed that dark iris colour was associated with more myopic refractive errors [4] may not come as much of a surprise. What is perhaps more surprising is that another study found that children with colour vision deficiencies (colour blindness) have a lower incidence of myopia than children with normal colour vision. [5]
What this shows is that there are many aspects of eye development which can affect the chances of developing myopia, and all of these are controlled by inherited characteristics.
So much so that one review commented that “Genes are estimated to explain up to 80% of the variance in refractive error.” [6]
However, despite this high percentage, we should not discount environmental factors. In fact, the more that scientists understand the genetic risk and the interdependence between genetic predisposition and environment, the better we can anticipate and plan to mitigate risk factors which may be within our control. In the meantime, early detection is key if we are to manage and perhaps slow down the development of myopia in our children.
In addition, Myopia Focus has launched a change.org campaign, which seeks to get the NHS to recognise myopia as an ocular disease and fund myopia management. Myopia onset usually occurs between six and thirteen years of age - earlier onset is associated with higher levels of myopia in adult life. 2.6 billion people in the world are myopic, and places like Singapore and South Korea have very high concentrations with 80 per cent or more of the people having myopia.