45,X: The Turner Syndrome Karyotype Explained
Last updated: October 22, 2026
45,X
45,X is the karyotype of a female with a single X chromosome and no second sex chromosome, the classic finding in Turner syndrome. This page explains the notation, how to confirm the result at the microscope, and why Turner referrals need extra attention to mosaicism.
What 45,X means
| Part | Meaning |
|---|---|
| 45 | 45 chromosomes in each cell analyzed, one fewer than the usual 46 |
| X | A single X chromosome and no second sex chromosome |
The missing sex chromosome is shown simply by listing one X, so the karyotype is written 45,X and not 45,X,−X. You may still see the older form 45,XO in textbooks, but it is not correct ISCN.
What to look for in the karyotype
- A count of 45 in every cell.
- 15 C-group chromosomes instead of the 16 a normal female has: the seven pairs 6 to 12 plus a single X.
- No Y chromosome in the G group: only two 21s and two 22s.
- A normal-looking X. Confirm that the remaining X is not itself abnormal, and that what looks like a missing X is not an X hidden in an overlap.
Look for mosaicism. Many people with Turner syndrome have a second cell line, such as 46,XX or a cell line with a structurally abnormal X or a Y. Laboratories usually count more cells than in a routine study when Turner syndrome is suspected, so that a low-level second cell line is not missed.
How 45,X happens
A 45,X karyotype results from the loss of a sex chromosome, either in the formation of an egg or sperm or in the early embryo. In most 45,X cases the remaining X comes from the mother, meaning the missing sex chromosome was the father's. Unlike the autosomal trisomies, 45,X is not associated with older maternal age.
Only about half of people with Turner syndrome have 45,X in all cells. The rest have mosaicism or a structurally abnormal X.
Related karyotypes
| Karyotype | Meaning |
|---|---|
| mos 45,X[15]/46,XX[15] | Mosaic Turner syndrome with a normal female cell line |
| 46,X,i(X)(q10) | An isochromosome of the long arm of X in place of a normal X |
| 46,X,del(X)(p11.2) | A deletion of the short arm of one X |
| 46,X,r(X)(p22.1q27) | A ring X chromosome |
| mos 45,X[20]/46,XY[10] | Mosaicism with a cell line carrying a Y chromosome |
Finding Y chromosome material matters clinically, because it raises the risk of a tumor called gonadoblastoma, and removal of the gonads is usually recommended. When a marker chromosome is present in a Turner karyotype, FISH is used to check whether it contains Y material.
About Turner syndrome
Turner syndrome affects roughly 1 in 2,000 to 2,500 live-born females. Common features include short stature, ovarian insufficiency leading to delayed puberty and infertility, and heart and kidney differences, particularly coarctation of the aorta and a bicuspid aortic valve. Some babies have a webbed neck or swelling of the hands and feet at birth. Intelligence is usually in the typical range. Growth hormone and estrogen therapy are often part of treatment.
Common questions
What is the difference between 45,X and 45,XO?
They describe the same thing. 45,X is the correct ISCN form, and 45,XO is an older way of writing it.
Why do labs count more cells for Turner syndrome?
Because mosaicism is common in Turner syndrome and affects the outlook and care, including whether a Y-bearing cell line is present. Counting more cells makes it less likely that a minor cell line is missed.
Keep learning
This page is for education and training. It is not medical advice and must not be used to diagnose or treat anyone. Laboratory results should be interpreted by qualified professionals following current standards.