Why do persons with Turner syndrome have developmental abnormalities, when normal XX-females do not, even though they only have 1 active X chromosome? From what I know, one X-chromosome is inactivated into a Barr body in normal XX-females, but these obviously don't show the symptoms of Turner syndrome. What function does the inactive chromosome have?
The reason is that X-inactivation is not complete (Carrell & Willard, 2005; Ahn & Lee, 2008), and as many as 15-25% of X-linked genes escape silencing (Carrell & Willard, 2005; Cheng et al, 2005). This means that some genes on the Barr body are expressed in XX-females, although often at lower levels compared to the active X-chromosome, and this is part of the normal gene dosage. The "purpose" of X-inactivation is usually dosage compensation, so that genes are expressed equally in males and females (and then actually upregulated in both sexes to balance autosomal genes, see e.g. Dementyeva et al, 2009). However, this is not true for all genes, and the ones that normally escape X-inactivation will be expressed in the incorrect dosage in X0 females. Some of these genes are thought to contribute to the symptoms of Turner syndrome.
It is also likely that the genes that escape silencing are involved in sex-linked traits, as stated in Carrell & Willard (2005):
Such genes are potential contributors to sexually dimorphic traits, to phenotypic variability among females heterozygous for X-linked conditions, and to clinical abnormalities in patients with abnormal X chromosomes.
However, as many of the genes that escape from inactivation do not have Y-linked homologues, strict dosage compensation may not be necessary for all genes on the chromosome. Such characteristic genomic differences should be recognized as a factor for explaining sex-specific phenotypes both in complex disease as well as in normal, sexually dimorphic traits.
Cheng et al (2005) also point out that the expression of the genes that escape silencing is tissue-dependent, which suggests that the role these genes play in the symptoms of Turner syndrome is also tissue dependent.
Very interesting question!
And I don't think either that the linked Wikipedia article answers the question.
Unfortunately I do not know the answer for sure, but I might have some relevant information to add. So, from my studies I know that Barr-bodies are not totally inactive, they, in fact, do have some very limited transcriptional activity. It might be low in intensity, but still vital? I'm just guessing.
My other guess is linked to the development of a Barr-body in a cell with two active XX chromosomes early during fetal development. So this happens via a process called X-inactivation in which a gene called Xist becomes active on both X chromosomes, then another gene, the Tsix also becomes active on one of the chromosomes and suppresses Xist on the other. The one with the suppressed Xist remains active, the other becomes the Barr-body (the process of selecting which chromosome becomes the Barr-body is random, hence female mosaicism). So, the point is, that for correct X-inactivation, the interplay of two X chromosomes are needed, and people with Turner syndrome have only one. That might be the problem, e.g. the Xist becomes active on the one X they have and starts interfering with the normal functioning of the chromosome, and there's no Tsix to suppress it.
But, again, I'm just making educated guesses here.