title: Ophthalmic phenotypes and the representativeness of twin data for the general population.
abstract: OBJECTIVE
  To compare the distributional parameters for a series of ocular biometric traits between twins and their singleton siblings, to evaluate the generalizability of twin data, as used in heritability analyses to the general population.
  
  
  METHODS
  A series of birth, anthropometric, and 13 ocular biometric traits were selected for analysis: interpupillary distance (IPD), visual acuity (logMAR), spherical equivalent refractive error, corneal curvature, axial length, anterior chamber depth (ACD), central corneal thickness (CCT), intraocular pressure (IOP), optic disc, cup and rim areas, and measures of retinal vessel caliber; central retinal arteriolar equivalent (CRAE), and central retinal venular equivalent (CRVE). Structural equation modeling was used to test the assumption that the means and variances for each trait did not differ between twins and their siblings.
  
  
  RESULTS
  Significant differences in log-likelihood for birth weight and gestational age were observed between twins and siblings, with the latter being both heavier and closer to full-term at birth. Siblings were also found to have larger IPD and axial length, and better visual acuity compared with their twin counterparts. Refractive error, corneal curvature, ACD, CCT, optic disc parameters, and retinal vascular calibers did not differ significantly between the two groups.
  
  
  CONCLUSIONS
  Twins are representative of the general population for some but not all measures of ocular biometry. Consequently, care should be taken when extrapolating twin data for these traits in heritability and other genetic studies. Birth weight differences between twins and siblings do not appear to account for the differences in ocular biometry observed in this study.
authors.affiliation: Centre for Eye Research Australia, Department of Ophthalmology, University of Melbourne, Royal Victorian Eye and Ear Hospital, East Melbourne, Victoria, Australia.
authors.forename: Paul G
authors.initials: PG
authors.lastname: Sanfilippo
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Medland
authors.forename: Alex W
authors.initials: AW
authors.lastname: Hewitt
authors.forename: Lisa S
authors.initials: LS
authors.lastname: Kearns
authors.forename: Jonathan B
authors.initials: JB
authors.lastname: Ruddle
authors.forename: Cong
authors.initials: C
authors.lastname: Sun
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Hammond
authors.forename: Terri L
authors.initials: TL
authors.lastname: Young
authors.forename: Nicholas G
authors.initials: NG
authors.lastname: Martin
authors.forename: David A
authors.initials: DA
authors.lastname: Mackey
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000867:Anterior Chamber / Q000033:anatomy & histology; D000886:Anthropometry / Q000379:methods*; D001699:Biometry / Q000379:methods*; D002648:Child; D002675:Child, Preschool; D003315:Cornea / Q000033:anatomy & histology; D005123:Eye / Q000033:anatomy & histology*; D005260:Female; D006801:Humans; D007231:Infant, Newborn; D007429:Intraocular Pressure; D008297:Male; D008875:Middle Aged; D010641:Phenotype; D011680:Pupil; D012160:Retina / Q000033:anatomy & histology; D012171:Retinal Vessels / Q000033:anatomy & histology; D014792:Visual Acuity; D055815:Young Adult