Embryonic Determination of Brain Programming Asymmetry - A Neglected Element in Twin-Study Genetics of Human Mental Development

Abstract

Departures from the usual human brain asymmetry, manifested in unusual laterality of motor dominance and/or speech representation, seem to have some genetic basis, and have been found associated with: schizophrenia, epilepsy, alcoholism, dyslexia, dysphasia, autism, and mental retardation. This can also derive from embryonic disturbances, as evidenced by excessive association with: first births, late births, birth difficulties, and twinning. MZ twins develop from embryos which have often split after at least some commitments to the cellular development of bilateral symmetry have been made. The effects of such a split are visibly recorded in mirror-imaging, in tooth emergence patterns, in shapes of mouth, nostrils, eyes, and ears, in hairwhorl placement and pattern. Brainfunction laterality is also found to be subject to mirror-imaging.Data from the 1972 publication of the Gottesman and Shields genetic twin study of schizophrenia serve to illustrate the power of the neglected embryonic discriminator: Fully one-third of their MZ schizophrenic sample {nearly ten times the best population estimate) were characterized as lefthanded or ambidextrous. If the sample is divided between pairs with and without any sign of laterality disturbance, the results are very strikingly altered. The pairs with no sign of laterality disturbance are 88% concordant, whereas unusual laterality pairs are 24% concordant. Several severity indicators are in excess, by ratios of 2 to 10-fold, among the RH pairs. Nuclear subtypes are (2.5) times as frequent, and schizoid premorbid personalities are twice as frequent, among the RH pairs.