Pol et al. have explored the genetic influence on focal gray matter (GM, nerve cell bodies) and white matter (WM, myelin covered axon tracts) densities in magnetic resonance brain images of 54 monozygotic and 58 dizygotic twin pairs and 34 of their siblings. To explore the common genetic origin of focal GM and WM areas with intelligence, they obtained cross-trait/cross-twin correlations in which the focal GM and WM densities of each twin are correlated with the psychometric intelligence quotient of his/her cotwin. They found genes to significantly influence WM density of the superior occipitofrontal fascicle, corpus callosum, optic radiation, and corticospinal tract, as well as GM density of the medial frontal, superior frontal, superior temporal, occipital, postcentral, posterior cingulate, and parahippocampal cortices. Moreover, their results showed that verbal (VIQ) and nonverbal (performance) (PIQ) intelligence quotient share a common genetic origin with an anatomical neural network involving the frontal, occipital, and parahippocampal GM and connecting GM of the superior occipitofrontal fascicle, and corpus callosum.
Figure legend: Cross-trait/cross-twin correlations for GM and WM density and VIQ/PIQ in MZ and DZ twin pairs ranging from 0 to 0.5. The cross-trait/cross-twin correlations were significant for GM density with VIQ in the right parahippocampal gyrus and for WM density with PIQ in the right superior occipitofrontal fascicle. A significant cross-trait/cross-twin correlation indicates that the genes influencing GM and WM density partly overlap with the genes that influence VIQ/PIQ. Note that, for illustration purposes, positive cross-correlations as shown here were not thresholded for significance. By definition, the cross-correlations in voxels that were not significantly determined by genetic factors could not become significant (because both factors, i.e., GM and WM density and VIQ and PIQ measures, have to be determined by genes to allow for inferences that possible mutual genes determine that association). Negative cross-correlations (data not shown) were present, but none of these reached significance.