Age trajectories of genetic variance in physical functioning: a longitudinal study of Danish twins aged 70 years and older. Behav Genet

Genetic-evolutionary theories of aging predict that the genetic variance for fitness traits increases with age, while epidemiological-gerontological theories predict an increase in the environmen-tal variance for most traits. In this study we examine the age trajectories of the genetic and en-vironmental variance in physical functioning in a sample of 4731 Danish twins aged 701 who are being followed longitudinally every second year with up to four assessments completed. A biometric growth model (Neale and McArdle, 2000) was applied to a validated physical ability score. The model included an overall level effect, a rate of linear change effect, and residual effects. The best-fitting model was a sex-specific model including additive genetic and nonshared environmental factors affecting level and rate of change and only nonshared environmental fac-tors affecting the wave-specific levels. For both sexes there is an approximate doubling of both the total variance and the genetic variance in the physical ability score over the four waves and, hence, a rather stable heritability. However, the heritability is approximately.10 for males and.30 for females in all four waves. The heritability of level and slope showed a similar pattern:.11–14 in males and.35–.39 in females. The increase in both additive genetic variance and en-vironmental variance is in agreement with genetic-evolutionary and epidemiological-geronto-logical theories of aging, respectively. The present study suggests that overall level of strength may be a better phenotype for future molecular genetic studies on physical functioning in the elderly than rate of change, because rate of change is vulnerable to sample attrition due to mor-tality and dropout and because four waves were needed to be able to detect a heritability for rate of change of the same magnitude as the heritability for level of physical functioning. KEY WORDS: Aging; twins; genetics; evolution; physical abilities; rate of change