not particpate were included in genetic analysis. The participants were predominantly of Northern European ancestry (>90%) and aged 26 to 89 years (mean age was 43.8 ± 11.5 years) at the time of testing. Subjects gave written informed consent and provided blood samples from which DNA was isolated using standard protocols. Genetic studies were approved by the QIMR Human Research Ethics committee. Zygosity of same-sex twins was assessed using nine polymorphic DNA microsatellite markers (AmpF1STR Profiler Plus Amplification Kit, Applied Biosystems, Foster City, CA) and three blood groups (ABO, MNS, and Rh), giving a probability of correct assignment greater than 99.99% (Nyholt, 2006a).
