Evidence of shared genes in refraction and axial length: the Genes in Myopia (GEM) twin study.

PURPOSE Axial length has been shown to explain up to 50% of the total variance in refraction, with axial length and refraction having a major genetic component. However, no study has attempted to determine whether the correlation between axial length and refraction is explained by shared genetic or environmental factors. METHODS All twins from Victoria aged 18 years or older were invited to participate in the Genes in Myopia (GEM) twin study through the Australian Twin Registry (ATR). Each twin completed a general questionnaire and underwent dilated objective refraction assessment and measurement of axial length. RESULTS A total of 612 twin pairs (1224 twins) aged from 18 to 86 years were examined in the GEM twin study. Axial length correlated negatively with refraction (r = -0.64 in the men, r = -0.68 in the women; P < 0.01). The sex limitation ADE (A, additive genetic; D, dominant genetic; E, unique environmental factors) model provided the best-fit genetic model for both measures. Of the variation in spherical equivalence in both the men and the women, approximately 50% were due to genetic factors influencing axial length. CONCLUSIONS From these findings, it is likely that axial length and refraction share common genes in their etiology. The GEM twin study has provided a basis and direction for future research into identifying the gene(s) in axial length that will ultimately improve our understanding of the etiology of refractive error, particularly myopia.