Aim: To investigate whether myopia is associated with a gene-environment interaction with environmental factors related to growth rate.

Hypothesis: We hypothesise that the genetic effect on myopia is moderated through an environmental interaction with environmental factors which affect childhood growth rate. This hypothesis is suggested by the following three observations: First, we note that myopia is common. Estimates of prevalence vary across countries and subpopulations, but a prevalence of around 30% in the US is a reasonable. Second, myopia is highly heritable. Heritability estimates vary, but twin studies have estimated heritability as high as 80% though population based estimates are lower. However, even the lower estimates suggest a significant genetic component. However, the clear evolutionary disadvantage of myopia makes it likely that much of the genetic component does not act directly, suggesting that we look for an interaction effect. Third, most cases of mild to moderate myopia are associated with elongation of the eye's axis relative to the optimum for its refractive power. This is effectively a problem with the shape of the eye which suggests a developmental or growth related background.

We plan to scan for genetic variants which are associated with myopia in the presence of environmental factors which affect growth. Specifically, we intend to use head circumference and other growth related phenotypes as a proxy measure for those environmental factors.