Sexual dimorphism in cognition and behaviour: the role of X-linked genes

Chimpanzees and humans last shared a common ancestor between 5 and 7 million years ago; 99% of the two species' DNA is identical. Yet, since the paths of primate evolution diverged, there have been remarkable developments in the behavioural and cognitive attainments of our species, which ultimately reflect subtle differences in gene structure and function. These modifications have occurred despite evolutionary constraints upon the diversity of genetic influences, on the development and function of neural tissue. Significant species differences can be observed both at the levels of function (gene expression) and structure (amino acid sequence). Protein evolution is driving an accelerating increase in brain complexity and size. Playing centre stage, in terms of the proportion of genes involved in brain development and cognitive function, is the X chromosome. Recently, it has become clear that a long-standing theory, implicating X-linked genes in a sexually antagonistic evolutionary role, is probably correct. Genes on the sex chromosomes can directly influence sexual dimorphism in cognition and behaviour, independent of the action of sex steroids. Mechanisms by which sex-chromosomal effects, due to X-linked genes, influence neural development or function are reviewed. These include the biased expression of genes subject to X-inactivation. haploin-sufficiency (in males) for non-inactivated genes with no Y homology, sex-specific brain functions and genomic imprinting of X-linked loci. Evidence supporting each of these mechanisms is available from both human and animal models. Recently, the first candidate genes have been discovered.