I want to understand how Turner syndrome - monosomy X - occurs at the molecular level. The NIH mentions that there are cases of both complete monosomy and of mosaicism.

I’m particularly interested in the monosomy case, as this really confuses me. How does a fertilized egg end up with only one sex chromosome (an X, in this case)? Is an empty egg fertilized by an X-bearing sperm? Is an X-bearing egg fertilized by an empty sperm?

The case of the mosaicism is easier to imagine - say through X chromosome nondisjunction at a very early point in some cells of the embryo. NB: unsure this is actually how mosaic monosomy X happens.

I haven't found a source that doesn't begin from the point at which monosomy has already occurred.

How do things get to that point? Where and how is a sex chromosome lost, such that a human can develop with only the one?

Thanks so much for any pointers!


1 Answer 1


Most often, it seems that Turner syndrome is due to fertilization by a sperm cell lacking a sex chromosome. I wouldn't describe such a cell as "empty": it still has the other 22 chromosomes, but in meiosis the sex chromosomes failed to segregate appropriately.

Monroy et al 2002 note that of 10 people with complete monosomy X, 9 had their single chromosome from the mother.

Monroy, N., López, M., Cervantes, A., García‐Cruz, D., Zafra, G., Canún, S., ... & Kofman‐Alfaro, S. (2002). Microsatellite analysis in Turner syndrome: parental origin of X chromosomes and possible mechanism of formation of abnormal chromosomes. American journal of medical genetics, 107(3), 181-189.

  • $\begingroup$ Thank you! I hadn't found that reference earlier. Definitely helps me to understand it better. $\endgroup$
    – Forest
    Jun 29, 2019 at 16:00

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