POWIC, Department of Psychiatry, Warneford Hospital, Oxford OX3 7JX, UK
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INTRODUCTION |
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GENETIC AND ENVIRONMENTAL INFLUENCES |
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What environmental factor might influence the development of cerebral asymmetry? Procopio points to the finding of Salvesen et al (1993) that children who had been screened by ultrasonography in utero were more likely to be non-right-handed than those who had not. However, the difference was small (odds ratio 1.32%; 95% CI 1.02-1.71). As Salvesen et al emphasise, non-right-handedness was one of six initial hypotheses, and no association with impaired neurological development was found. An effect of ultrasound on cerebral dominance is a concern but the evidence is modest.
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NATURE AND LOCATION OF THE ASYMMETRY FACTOR |
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I have argued (Crow, 1993, 1994) that the pattern of verbal and spatial deficits associated with the sex chromosome aneuploidies indicates that the genetic determinant of asymmetry is in a region of homology between the X and the Y chromosomes. The evolutionary history of the sex chromosomes provides a pointer to its location (Lambson et al, 1992; Sargent et al, 1996, 2001; Schwartz et al, 1998): after the separation of the lineages that led to the chimpanzee and H. sapiens a translocation occurred from Xq21.3 to the Y chromosome short arm, and the translocated block was split by a subsequent paracentric inversion (Fig. 3).
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Gene sequences within this block are present on the X and Y chromosomes in humans but only on the X in other primates. Within this region of homology a gene protocadherinXY has recently been described (Blanco et al, 2000) that is a member of a class of cell adhesion molecules expressed in the brain that have a role in axonal guidance. It is therefore a candidate for H. sapiens-specific characteristics such as cerebral asymmetry (Crow, 2001). In that there are sequence differences between the X and Y copies, this gene can account for gender differences such as those observed in age of onset of psychosis, lateralisation and the development of verbal ability. According to the XY theory of cerebral asymmetry, the chromosomal re-arrangements that led to protocadherinX being represented on the Y as well as the X chromosome were speciation events in hominid evolution (Crow, 2000).
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EPIGENETICS OF ASYMMETRY |
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CONCLUSIONS |
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REFERENCES |
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Annett, M. (1999) The theory of an agnosic right shift gene in schizophrenia and autism. Schizophrenia Research, 39, 177-182.[CrossRef][Medline]
Bickerton, D. (1995) Language and Human Behavior. Seattle, WA: University of Washington.
Blanco, P., Sargent, C. A., Boucher, C., et al (2000) Conservation of PCDHX in mammals; expression of human X/Y genes predominantly in the brain. Mammalian Genome, 11, 906-914.[CrossRef][Medline]
Burgoyne, P. S. (1982) Genetic homology and XY crossing over in the X and Y of mammals. Human Genetics, 61, 85-90.[Medline]
Burgoyne, P. S. & McLaren, A. (1985) Does XY pairing during male meiosis protect the paired region of the X chromosome from subsequent X-inactivation? Human Genetics, 70, 82-83.[Medline]
Crow, T. J. (1991) Protection from X inactivation. Nature, 353, 710.[Medline]
Crow, T. J. (1993) Sexual selection, Machiavellian intelligence and the origins of psychosis. Lancet, 342, 594-598.[Medline]
Crow, T. J. (1994) The case for an XY homologous determinant of cerebral asymmetry. Cytogenetics and Cell Genetics, 67, 393-394.
Crow, T. J. (1998a) Sexual selection, timing and the descent of Man: a genetic theory of the evolution of language. Current Psychology of Cognition, 17, 1079-1114.
Crow, T. J. (1998b) Why cerebral asymmetry is the key to the origin of Homo sapiens: how to find the gene or eliminate the theory. Current Psychology of Cognition, 17, 1237-1277.
Crow, T. J. (2000) Did Homo sapiens speciate on the Y chromosome? Psycholoquy, 11, 1-18.[CrossRef]
Crow, T. J. (2001) ProtocadherinXY: a candidate gene for cerebral asymmetry and language. In The Transition to Language (ed. A. Wray), pp. 93-112. Oxford: Oxford University Press.
Crow, T. J., Done, D. J. & Sacker, A. (1995) Childhood precursors of psychosis as clues to its evolutionary origins. European Archives of Psychiatry and Clinical Neuroscience, 245, 61-69.[Medline]
Crow, T. J., Done, D. J. & Sacker, A. (1996) Cerebral lateralization is delayed in children who later develop schizophrenia. Schizophrenia Research, 22, 181-185.[CrossRef][Medline]
Crow, T. J., Crow, L. R., Done, D. J., et al (1998) Relative hand skill predicts academic ability: global deficits at the point of hemispheric indecision. Neuropsychologia, 36, 1275-1282.[CrossRef][Medline]
Jegalian, K. & Page, D. C. (1998) A proposed mechanism by which genes common to mammalian X and Y chromosomes evolve to become X inactivated. Nature, 394, 776-780.[CrossRef][Medline]
Lambson, B., Affara, N. A., Mitchell, M., et al (1992) Evolution of DNA sequence homologies between the sex chromosomes in primate species. Genomics, 14, 1032-1040.[Medline]
Leask, S. J. & Crow, T. J. (2001) Word acquisition reflects lateralization of hand skill. Trends in Cognitive Sciences, 5, 513-516.[CrossRef][Medline]
Lyon, M. F. (1974) Evolution of X-chromosome inactivation in mammals. Nature, 250, 651-653.[Medline]
Lyon, M. F. (1999) X-chromosome inactivation. Current Biology, 9, 235-237.
Marchant, L. F. & McGrew, W. C. (1996) Laterality of limb function in wild chimpanzees of Gombe National Park: comprehensive study of spontaneous activities. Journal of Human Evolution, 30, 427-443.[CrossRef]
Mellars, P. (1998) Modern humans, language, and the symbolic explosion. Cambridge Archaeological Journal, 8, 88-94.
Orton, S. T. (1937) Reading, Writing and Speech Problems in Children. New York: Norton.
Procopio, M. (2001) Handedness and
schizophrenia: genetic and environmental factors (letter). British
Journal of Psychiatry, 179,
75-76.
Provins, K. A., Milner, A. D. & Kerr, P. (1982) Asymmetry of manual preference and performance. Perceptual and Motor Skills, 54, 179-194.
Salvesen, K. A., Vatten, L. J., Eik-Nes, S. H., et al (1993) Routine ultrasonography and subsequent handedness and neurological development. BMJ, 307, 159-164.[Medline]
Sargent, C. A., Briggs, H., Chalmers, I. J., et al (1996) The sequence organization of Yp/proximal Xq homologous regions of the human sex chromosomes is highly conserved. Genomics, 32, 200-209.[CrossRef][Medline]
Sargent, C. A., Boucher, C. A., Blanco, P., et al (2001) Characterization of the human Xq21.3/Yp11 homology block and conservation of organization in primates. Genomics, 73, 77-85.[CrossRef][Medline]
Schwartz, A., Chan, D. C., Brown, L. G., et al
(1998) Reconstructing hominid Y evolution: X-homologous
block, created by XY transposition, was disrupted by Yp inversion
through LINELINE recombination. Human Molecular
Genetics, 7,
1-11.
Sommer, I., Aleman, A., Ramsey, N., et al
(2001) Handedness, language lateralisation and anatomical
asymmetry in schizophrenia. Meta-analysis. British Journal of
Psychiatry, 178,
344-351.
Steele, J. (1998) Cerebral asymmetry, cognitive laterality and human evolution. Current Psychology of Cognition, 17, 1202-1214.
Steinmetz, H., Herzog, A., Schlaug, G., et al (1995) Brain (a)symmetry in monozygotic twins. Cerebral Cortex, 5, 296-300.[Abstract]
Stringer, C. & McKie, R. (1996) African Exodus: The Origins of Modern Humanity. London: Jonathan Cape.
Received for publication September 11, 2001. Revision received January 30, 2002. Accepted for publication May 7, 2002.