The importance of genes and environment for ocular refraction and its determiners: a population based study among 20-45 year old twins

Aims-To estimate the heritability for ocular refraction and its determiners in a population based cohort of 20-45 years old twins. Methods-114 twin pairs (53 monozygotic and 61 dizygotic) participated. Refr action was determined in cycloplegia and eye dimensions were measured with ultrasound. Educational length was assessed. The heritability was estimated employing aetiological model fitting. Evidence of gene-enviromnent interac tion was analysed. Correlations between intrapairwise differences in educat ional length and in refraction were evaluated. Results-The heritability was between 0.89 and 0.94 (95% CI: 0.82, 0.96) for refraction, total refraction, axial length, and radius of corneal curvatur e. Phenotypic variation was mostly due to additive genetic effects. Refract ion revealed evidence of gene-environment interaction (r = -0.29 to -0.32; p <0.05). The heritability for anterior chamber depth and lens thickness wa s between 0.88 and 0.94 (95% CI: 0.81, 0.96) and dominant genetic effects w ere the most likely explanation. There was no correlation between age and i ntrapairwise differences in refraction. The dizygotic twins had significant larger intrapairwise differences in educational length (p <0.05), but the differences were not correlated with differences in refraction. Conclusions-The results indicate a high heritability for ocular refraction and its determiners and thus suggest that environmental impact on refractio n is not significant. However, the epidemiological association between educ ational length (near work) and myopia, the evidence of increasing myopia pr evalence within a few generations, and the theory of gene-environment inter action imply that some individuals might be genetically liable to develop m yopia if exposed to certain environmental factors.