Until recently it had been assumed that differences in gene activity of the right and left hemisphere of the brain might be responsible for a person's handedness. A preference for moving the left or right hand develops in the womb from the eighth week of pregnancy, according to ultrasound scans. From the 13th week of pregnancy, unborn children prefer to suck either their right or their left thumb.
The motor cortex in the brain is responsible for initiating arm and hand movements. It sends a corresponding signal to the spinal cord, which in turn translates the command into a motion. New research now shows that the motor cortex is not connected to the spinal cord at an early stage in the womb. However, even before the connection forms, precursors of handedness become apparent. This new finding has given strength to the argument that left or right preference must be rooted in the spinal cord, rather than in the brain.
The researchers analyzed the gene expression in the spinal cord during the eighth to twelfth week of pregnancy and detected marked right-left differences in the eighth week — in precisely those spinal cord segments that control the movements of arms and legs.
The researchers traced the cause of asymmetric gene activity to epigenetic factors reflecting environmental influences. Epigenetic factors are compounds that attach to, or ‘mark’ DNA and interact with genetic material without changing the underlying DNA sequence. They act as chemical tags, indicating and influencing what, where, and when genes should be ‘turned on’ or expressed. Those influences might, for example, lead to enzymes bonding methyl groups to the DNA, which in turn would affect and minimize the reading of genes. As this occurs to a different extent in the left and the right spinal cord, there is a difference to the activity of genes on both sides.