Distribution and heritability of iris thickness and pupil size in Chinese: the Guangzhou Twin Eye Study.

PURPOSE To investigate the distribution and heritability of iris thickness and pupil diameter measured by anterior segment optical coherence tomography (ASOCT) in a classic twin study. METHODS Twins aged 8 to 16 years were enrolled from the Guangzhou Twin Registry. ASOCT was used to obtain one horizontal scan, and the images were analyzed with custom software. Iris thickness was measured at 750 microm (IT750) and 2000 microm (IT2000) from the scleral spur, as well as cumulative iris area (IAREA). Pupil diameter in millimeters (PDMM) was measured between the most central points of the iris at the nasal and temporal sides. Zygosity was confirmed by genotyping in all same-sex twin pairs. Heritability was assessed by structural variance component genetic modeling after adjustment for age and sex with the Mx program. RESULTS Participating in the analysis were 309 monozygotic (MZ) and 165 dizygotic (DZ) twin pairs. The mean IT750, IT2000, and IAREA were 0.406+/-0.075 (SD) mm, 0.514+/-0.075 mm, and 1.169+/-0.184 mm(2), whereas the PDMM was 5.600+/-0.0821 mm. The intraclass correlation coefficients for the IT750, IT2000, IAREA, and PDMM were 0.53, 0.54, 0.61, and 0.63 for MZ pairs and 0.29, 0.11, 0.35, and 0.34 for DZ pairs, respectively. A model with additive genetic and unique environmental effect was the most parsimonious, and the heritability was approximately 60% for iris thickness and pupil diameter. CONCLUSIONS Iris thickness is thinner in girls but is not associated with age in children. The variation is largely attributable to additive genetic effects.