150 Maternal Twins and Male Gender Bias in Autism Spectrum Disorders
monozygotic twins are of opposite handedness [19,20] with some even show-
ing reversed cerebral asymmetries [21,22]. They evaluated arcuate fasciculus
morphology in 35 pairs of monozygotic twins, of which 17 pairs were concord-
ant for handedness and 18 pairs were discordant for handedness. Of note, the
presence of arcuate fasciculus appears to be unique to humans and its con-
nectivity has been shown to correspond to various functional areas within the
temporal, parietal, and frontal lobes. Furthermore, this region of the brain
plays an important role in language and speech development. Functional
hemispheric language dominance was established for each twin member using
functional magnetic resonance imaging (fMRI) that showed 26 twin pairs con-
cordant and 9 twin pairs discordant for language dominance. Their data sug-
gest that handedness and hemispheric dominance for speech production might
be influenced by environmental factors.Differences in Frontal and Limbic Brain Activation in Monozygotic
Twin Pairs Discordant for Severe Stressful Life Events
Godinez et al. [23] looked at the discordance in maternal twins who suffered
severe stressful life events during early development (i.e., before 18 years of
age) and looked at their brain activation during performance of an emotional
word‐face task (using functional MRI). Each of the twins was discordant in
exposure to severe stress such that one twin who had suffered two or more
severe events exhibited significant clusters of greater activation in the ventro-
lateral and medial prefrontal cortex, basal ganglia, and limbic regions as com-
pared with their control co‐twin who had suffered no severe events.Structural Connectivity of the Brain of a Child with ASD and That
of the Unaffected Identical Twin
Conti et al. [24] studied the structural connectivity of the brain of a child with
ASD, and of that of his unaffected identical twin, using high angular resolution
diffusion imaging (HARDI) that can show neuronal tracks in brains. Around 11%
of the discordant intra‐hemispheric tracts showed lower fractional anisotropy
(FA) values in the ASD twin, while only 1% showed higher values. This difference
was significant. Our findings in a disease‐discordant identical twin pair confirm
previous literature consistently reporting lower FA values in children with ASD.Differences in Genomic and Epigenomic Expression
in Monozygotic Twins Discordant for Rett Syndrome
Tran and Miyake [25] reported the genomic and epigenomic sequences in skin
fibroblasts of a discordant monozygotic twin pair with Rett syndrome. As we
have mentioned previously, this X‐linked neurodevelopmental disorder is