Differential methylation of the X-chromosome is a possible source of discordance for bipolar disorder female monozygotic twins.

Monozygotic (MZ) twins may be subject to epigenetic modifications that could result in different patterns of gene expression. Several lines of evidence suggest that epigenetic factors may underlie mental disorders such as bipolar disorder (BD) and schizophrenia (SZ). One important epigenetic modification, of relevance to female MZ twins, is X-chromosome inactivation.

The genetic basis for sex differences in human behaviour: role of the sex chromosomes.

The nature of the mechanisms underlying observed sex differences in human behaviour continues to be debated. This review concentrates on the thesis that genes on the sex chromosomes other than those directly controlling sex determination, and whose functions are, at least in part, independent from hormonal influences, play a significant role in determining gender differences in behaviour. To provide an adequate basis for examining this issue, the current understanding of the nature of sex determination, differences in behaviour and the influences of sex hormones are evaluated.

X inactivation as a source of behavioural differences in monozygotic female twins.

Although members of monozygotic twin pairs are identical in genome sequence, they may differ in patterns of gene expression. One early and irreversible process affecting gene expression, which can create differences within pairs of female monozygotic twins, is X inactivation - one twin can express mainly paternally-received genes on the X chromosome while the other twin expresses mainly maternally-received genes. It follows that non-identical X chromosome expression may cause female monozygotic twins to correlate less strongly than male monozygotic twins on complex behavioural traits affected by X-linked loci.