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M. Dirani, S. N. Shekar, P. N. Baird; Shared Genes Influence Both Refraction and Axial Length - The Genes in Myopia (GEM) Twin Study. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5424.
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Axial length and refraction have been shown to be significantly correlated, with axial length explaining up to 50% of the total variance in refraction. Twin and family studies have shown that axial length and refraction have a major genetic component. However, to our knowledge there has been no study that has determined whether the correlation between axial length and refraction is in part explained by shared genetic and environmental factors.
All twins from Victoria aged 18 years or older were invited to participate in this study through the Australian Twin Registry (ATR). Each twin completed a general questionnaire, vision examination, dilated objective refraction assessment and axial length measurements were obtained.
A total of 612 twin pairs (1224 twins) aged between 18-86 years (mean age = 52.36 years) were recruited and examined in this study. Axial length was significantly correlated with refraction (r = 0.64 in males, r = 0.68 in females), with axial length explaining more than 40% of the variance in refraction for both genders. All measured variables fitted a normal distribution. Heritability estimates for refraction and axial length were as high as 88% and 90% in males, respectively. The sex limitation ADE (A - additive genetic, D - dominant genetic, U - unique environmental factors) model provided the best-fit genetic model for both measures. Of the variation in spherical equivalence in males, 23% and 27% were due to additive genetic and non-additive genetic factors influencing axial length. For the same variation in females, the proportion explained by factors influence axial length were 28% and 25% for additive genetic and non-additive genetic influences respectively.
From these findings, we may postulate that axial length and refraction may share common genes in their aetiology.
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