In order to identify brain structural phenotypes that remain under signific
ant genetic control in late adulthood, we examined the heritability of corp
us callosum macrostructure (i.e. size) using MRI and microstructure (e.g. m
yelin) using diffusion tensor imaging in 15 monozygotic and 18 dizygotic tw
in pairs of elderly men. The relative proportion of genetic to environmenta
l influences varied considerably by region and structural type and was 5:1
for callosal macrostructure, 3:1 for splenium microstructure, and 1:1 for g
enu microstructure. This is the first in vivo identification of quantifiabl
e phenotypes of brain white matter microstructure and demonstrates signific
ant and differential genetic regulation in old age, with anterior interhemi
spheric connecting pathways more susceptible than posterior pathways to env
ironmental influences. NeuroReport 12:1677-1681. (C) 2001 Lippincott Willia
ms & Wilkins.