Twin studies, genome-wide association studies and myopia genetics

Myopia is an increasingly prevalent, complex trait influenced by numerous genetic and environmental factors. Whilst the increasing prevalence may be underlined by changing environmental pressures, the majority of trait variance is explained by genetic risk. In other words, the dispersion or variation in refractive error within individuals of a defined population is largely due to the effect of genes. This is supported by the crude but well-replicated dose-response association of one or more myopic parents and risk of myopia in their child (1-4), and to a better degree the high estimates of heritability for refractive error from twin studies (5-8). Theoretically, a genetic risk score containing all the genes contributing to myopia risk would explain the majority of the heritability and trait variance, whilst also providing a very useful predictive test. However, to date the identified genetic loci associated with refractive error explain only a small proportion of the estimated heritability, as discussed below.