Genetics

Turner's syndrome is the result of a sporadic chromosomal abnormality. It can result from the non-disjunction of chromosomes in either parent. Over 99% of 45X0 foetuses abort. The risk of recurrence is low.

A minority of individuals are mosaics 45X0/46XX, or more rarely 45XO/45XY.

Turner's syndrome is the only viable monosomy in humans. This is probably because in normal females one copy of the X chromosome in each cell is functionally inactivated in a process called lyonisation - leading to formation of the Barr body, the absence of which is a diagnostic feature in Turner's. Thus the normal situation in a cell is to have just one functional X chromosome; it is only in certain tissues that the loss matters.

Reference

1. Viuff M, Skakkebaek A, Nielsen MM, et al. Epigenetics and genomics in Turner syndrome. Am J Med Genet C Semin Med Genet. 2019 Mar;181(1):68-75.