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F. Carbonaro, T. Andrew, D. A. Mackey, T. D. Spector, C. J. Hammond; The Effect of Repeated Measurements on Heritability: A Twin Study. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5429.
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Heritability (h2) is the amount of variation explained by genetic factors; the rest being explained by environmental factors. h2 values are often used to estimate the statistical power of planned linkage or association studies. The environmental component is known to be affected by measurement error; the higher the measurement error, the lower the h2. We set out to determine whether repeated measurements had an effect on the outcome of the heritability of two separate parameters measured by two different instruments.
Monozygotic (MZ) and dizygotic (DZ) twin pairs from the TwinsUK Adult Twin Registry, were examined using 2 types of tonometer; the Goldmann Tonometer and the Pascal-Dynamic Contour Tonometer (DCT). Two readings per eye were taken with each instrument. A separate parameter measured by each instrument was chosen for the analysis. The parameters used were Intraocular Pressure (IOP) using the Goldmann and Ocular Pulse Amplitude (OPA) using the DCT. h2 of these parameters was calculated using a structural equation modelling program (Mx). The h2 of each parameter was compared when one, two and four readings were used in the analysis.
416 pairs of twins (211 MZ, 205 DZ) were examined, with a mean age of 53 years (range 16-81). Most were female (91.9%) and all were Caucasian. Mean IOP was 15.3mmHg (SD 2.7) and mean OPA was 2.8mmHg (SD 0.9). The h2 of IOP measured by Goldmann increased with one: 0.54 (95% CI: 0.45-0.61), two: 0.55 (95% CI: 0.45-0.63) and four: 0.6 (95% CI: 0.52-0.67) readings. The h2 of OPA also increased with one: 0.5 (95% CI: 0.38-0.6), two: 0.65 (95% CI: 0.55-0.74) and four: 0.68 (95% CI: 0.58-0.76) readings.
Heritability increases with increasing number of readings of IOP, by reducing measurement error. Future genetic studies may increase power to detect genetic associations by using the mean of several readings for both eyes, rather than a single reading from one eye.
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