A study published in the journal Investigative Ophthalmology and Visual Science finds that the identical genetic code of identical twins means that they have identical eyes, even down to the smallest details.
Over recent years, it has become clear that some of the larger defects in vision – such as myopia (nearsightedness) and astigmatism (an imperfection in the eye’s curvature) – have a strong genetic component.
Studies have shown that around 80 percent of the variance in myopia and 50 percent of variance in astigmatism is genetic in origin.
Other than the larger, often correctable aberrations in the eye, there are many more minor errors that occur in the visual system.
In general, the eye compensates for these small defects without the individual suffering any reduction in visual clarity. For instance, aberrations in the cornea are often compensated for by opposite aberrations in the lens.
Although there are many types of smaller optical defects, or higher order aberrations, the three main types that are most noticeable in everyday life are:
- Spherical aberration: These cause night myopia, in which the individual finds seeing in low light difficult. They may also see halos around bright objects.
- Coma: These are points of light, such as stars, that appear to have a tail similar to a comet.
- Trefoil: These points of light appear to be smeared in three directions, much like the Mercedes-Benz symbol.
Currently, little is known about the origin of higher order aberrations, but many researchers consider them to be a consequence of environmental factors, rather than our DNA.
Recently, researchers from Anglia Ruskin University’s Vision and Eye Research Unit, in the United Kingdom, teamed up with colleagues from the Laboratorio de Optica and the Murcia Twin Registry from the University of Murcia, in Spain. They set out to investigate how much of these higher order aberrations are due to genetics alone.
For the study, they recruited 69 sets of twins, 36 of whom were identical, or monozygotic. The team, headed up by Dr. Juan Tabernero, looked for optical defects in each eye.
They found that identical twins, with identical DNA, also had identical defects – even in individuals over the age of 50. This was a surprising finding as these people had been “using” their eyes for half a century, leaving them open for environmental factors – including diet, accidents, and disease – to make subtle changes.
In the nonidentical twins, who share an average of 50 percent of their genes, the ocular defects were different. This demonstrates that genetics play a much more substantial role than the environment when it comes to small changes in vision.
“We have shown how tiny optical defects of the eye that respond to imperfections as small as a few micrometers in the lenses of the human eye are extremely similar in identical twins, and not in nonidentical twins.”
Dr. Juan Tabernero
The results fly in the face of older theories, many of which presumed that environmental factors were mostly involved in optical aberrations.
With a charmingly romantic turn of phrase, Dr. Tabernero explains, “Aberrations are, for instance, responsible for how everyone sees stars in the sky slightly different at night. But now, we can say that only identical twins see identical stars.”
In the future, the authors hope that a large, genome-wide association study might be undertaken to find the genetic code beneath these small changes. Understanding the genes that underpin these minor changes might help us to understand the processes behind the larger errors found in some people’s eyes.