Why is Turner syndrome a problem?

Question

Turner syndrome is a chromosomal disorder where one of the X chromosomes is missing. For more details visit the Wikipedia page

I don't understand why missing one X chromosome would be harmful. In normal females one of the two X chromosomes is inactivated at an early embryonic stage to a Barr body. For more details visit the Wikipedia page on this.

This X chromosome inactivation can be beneficial. For example, women are less prone to sex-linked genetic disorders due to this X chromosome inactivation.

My questions are:

1. If only one copy of an X chromosome is active in normal human females, why would having only one copy of an X chromosome cause a disorder?

2. What does it mean to deactivate parts of a chromosome

3. Why would it be essential to have two copies of a gene, as only the dominant one is expressed?

Answer 1

Short answer
Some genes on the X chromosome escape X-inactivation. Two copies of these genes are needed for normal development. These genes are also present on the Y chromosome. Hence, healthy males and females both have two copies of these genes. In Turner's, the SHOX gene seems to be one of the culprits, which is needed for normal skeletal development.

Background
It appears that both of the X chromosomes are needed for normal development in females, as some genes on the X chromosome escape X-inactivation. Many of these genes are located at the ends of each arm of the X chromosome in areas known as the pseudoautosomal regions. Although many genes are unique to the X chromosome, genes in the pseudoautosomal regions are present on both sex chromosomes. As a result, men and women each have two functional copies of these genes. Many of these genes are essential for normal development (source: Genetics Home Reference).

Which genes on the X chromosome are responsible is not yet known, but a gene called SHOX seems to be involved, which is important for bone development and growth. The loss of one copy of this gene likely causes short stature and skeletal abnormalities in women with Turner syndrome (source: NIH), see Fig. 1.

SHOX is a transcription factor of the homeobox family. It acts during early embryonic development to control the formation of the development of the skeleton. It plays an important role in the growth and maturation of bones in the arms and legs source: NIH).

^{Fig. 1. Some developmental malformations seen in Turner's syndrome. source: University of Chicago}

_{FYI - I wrote an answer before checking for duplicates. I think my answer does add additional information (the SHOX gene is not mentioned in the linked question's answer}

Answer 2

@AliceD's answer is excellent, but I thought it would be helpful to include an additional perspective.

The issue here isn't that monosomy X shouldn't be a problem because the majority of one X chromosome is inactivated. It is that monosomy X isn't as big a problem as any other monosomy because the majority of one X chromosome is inactivated. Monosomies generally lead to embryonic or fetal loss. There are a number of other disorders and reports that are described as monosomies, but they are partial monosomies (e.g., monosomy 10, monosomy 22). Monosomy X is the only complete monosomy that regularly produces a viable fetus, occuring in about 1 in 4000 live female births. It is believed to be present in 1-2% of all conceptuses.

There is an excellent overview of this topic in Chapter 5 of Thompson & Thompson Genetics in Medicine, with a particular discussion of Turner syndrome in Chapter 6. There is also a review of the process of X inactivation, which involves usual strategies of silencing genes (conversion of a section of a chromosome to heterochromatin by covalent modifications of histone and DNA). It is just used more broadly on the X chromosome.