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- Why do patients with TS have phenotypic abnormalities despite the fact that
only one X chromosome is active in a normal female?
In a normal female, one of the X chromosomes undergoes inactivation (lyoniza-
tion); however, this inactivation is not complete and some genes escape lyoniza-
tion, which are essential for survival and development of a normal female.
These include genes present in the pseudoautosomal region (e.g., SHOX) and
some genes in other parts of the X chromosome (10–15 %). Patients with TS
(45,XO) have only one X chromosome, and the absence of those genes which
escape lyonization in the second X chromosome results in phenotypic abnor-
malities of TS.- Is 45,XO compatible with life?
It is known that 3 % of all conceptuses are 45,XO and 99 % of these are aborted.
This shows that the presence of both copies of X chromosomes is required for
survival. However, 1 % of fetuses with a karyotype of 45,XO are still able to
survive, and this can be explained by the presence of occult 45,XO/46XX
mosaicism.- How common is 45,XO karyotype in Turner syndrome?
Approximately 45–60 % of individuals with Turner syndrome have 45,XO
karyotype. 45,XO/46,XX mosaicism is the next common karyotypic
abnormality, which is present in 20–30 % of patients. Other abnormalities like
deletion of short arm (p-), isochromosome q, ring chromosome, and marker
chromosome contribute to the rest.- What is ring chromosome?
Ring chromosome is a structurally abnormal chromosome resulting from fusion
of both arms of a chromosome, after the breakage of genetic material from both
the distal ends. Ring chromosome [46,Xr(X)] is the cause of Turner syndrome
in approximately 10 % of patients. The presence of ring chromosome in Turner
syndrome (TS) is associated with spontaneous menarche in one-third, higher
incidence of mental retardation, and lower incidence of congenital malforma-
tions. The figure illustrated below shows formation of ring chromosome
(Fig. 8.5).8 Turner Syndrome