Turner's Syndrome (45, X0): Cause, Karyotype and Features
Biology · Principles of Inheritance and Variation · NEET
Turner's syndrome happens when a person is MISSING one X chromosome, so they have only 45 chromosomes with a single X (written 45, X0). This person is female but is sterile because her ovaries are not fully formed (rudimentary), and she lacks some secondary sexual features. Memory hook: "Turner = one X TURNED off" — only one X is left (45, X0), so she is a sterile female.
A normal female has two X chromosomes (46, XX). In Turner's syndrome one X chromosome is missing, giving 45, X0 — a sterile female with rudimentary ovaries.
Your doubts, answered
Is Turner's syndrome a male or a female?
It is a FEMALE. Even though one X chromosome is missing, one X is still present, and there is NO Y chromosome. A person with one X and no Y develops as a female. But she is a sterile female because her ovaries are rudimentary (not fully developed), so she cannot have children. Remember: no Y means female.
Is Turner's syndrome caused by an EXTRA chromosome or a MISSING one?
It is caused by a MISSING chromosome. One of the two X chromosomes is absent. So instead of the normal 46 chromosomes (44 autosomes + XX), a Turner female has only 45 chromosomes (44 autosomes + a single X). This is called monosomy of X. Do not confuse this with Down's and Klinefelter's, which are caused by an EXTRA chromosome.
What does 45, X0 mean? Why 45 and not 46?
Normal humans have 46 chromosomes (23 pairs). '45' means one chromosome is missing. 'X0' means only ONE X chromosome is present and its partner is absent (the '0' means nothing is there). So 45, X0 = 44 autosomes + 1 X chromosome = 45 total. NCERT writes it as '45 with X0'.
Why is a Turner female sterile?
Because her ovaries are RUDIMENTARY, meaning they did not develop properly. Ovaries make eggs and female hormones. If they are not fully formed, she cannot produce eggs, so she is sterile. She also lacks some secondary sexual characters (features that normally develop at puberty).
How is Turner's syndrome different from Klinefelter's syndrome?
Turner's syndrome = 45, X0 = one X MISSING = a sterile FEMALE. Klinefelter's syndrome = 47, XXY = one EXTRA X added to XY = a sterile MALE with some female features like gynaecomastia (breast development). Easy way: Turner takes away (45), Klinefelter adds (47).
What kind of disorder is this — Mendelian or chromosomal?
It is a CHROMOSOMAL disorder, not a Mendelian (single-gene) disorder. It is caused by the absence of a whole chromosome (aneuploidy), usually due to non-disjunction (failure of chromosomes to separate) during meiosis. Sickle cell anaemia and haemophilia are gene disorders; Turner's, Down's and Klinefelter's are chromosomal disorders.
⚠️ The NEET trap ✗ Turner's syndrome is caused by the presence of an ADDITIONAL X chromosome. ✓ Turner's syndrome is caused by the ABSENCE of one X chromosome (45, X0). It is Klinefelter's syndrome and Down's syndrome that are caused by an EXTRA chromosome. 🧠 NTA loves to SWAP the causes. If a statement says Turner = extra chromosome, it is WRONG. Turner = missing (45); Klinefelter/Down = extra (47/47). ReNEET 2026 asked exactly this swap.
Real NEET questions
NEET 2019 Odisha
Match the items of column I with column II:
Column I: (a) XX-XO method of sex determination (b) XX-XY method of sex determination (c) Karyotype-45 (d) ZW-ZZ method of sex determination
Column II: (i) Turner's syndrome (ii) Female heterogametic (iii) Grasshopper (iv) Female homogametic
A · a-ii, b-iv, c-i, d-iii
B · a-i, b-iv, c-ii, d-iii
C · a-iii, b-iv, c-i, d-ii ✓
D · a-iv, b-ii, c-i, d-iii
Solution: Karyotype 45 means one chromosome is missing, i.e., 45, X0 = Turner's syndrome, so c-i. XX-XO sex determination is seen in grasshopper (a-iii). In XX-XY type the female is homogametic (b-iv). ZW-ZZ (birds) has female heterogametic (d-ii). This gives a-iii, b-iv, c-i, d-ii.
ReNEET 2026
Statement I: Down's syndrome is caused by the absence of one of the X-chromosomes. Statement II: Turner's syndrome is caused by the presence of an additional copy of the chromosomes. Choose the correct answer:
A · Both Statement I and Statement II are correct
B · Both Statement I and Statement II are incorrect ✓
C · Statement I is correct but Statement II is incorrect
D · Statement I is incorrect but Statement II is correct
Solution: The two causes are swapped, so both statements are wrong. Down's syndrome is caused by an ADDITIONAL copy of chromosome 21 (trisomy 21), not a missing X. Turner's syndrome is caused by the ABSENCE of one X chromosome (45, X0), not an extra chromosome. Both statements are incorrect.
NEET 2019 Odisha
In which genetic condition, each cell in the affected person, has three sex chromosomes XXY?
A · Thalassemia
B · Klinefelter's Syndrome ✓
C · Phenylketonuria
D · Turner's Syndrome
Solution: Three sex chromosomes XXY (karyotype 47, XXY) is Klinefelter's syndrome, caused by an extra X. Turner's syndrome is the opposite — a MISSING X (45, X0) with only one sex chromosome. So the XXY condition is Klinefelter's, not Turner's. This contrast is a common NEET trap.
Solved Principles of Inheritance and Variation NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The karyotype is 45, X0. This means 45 total chromosomes: 44 autosomes plus a single X chromosome. One X chromosome is absent (monosomy of X).
How many chromosomes does a Turner's syndrome person have?
45 chromosomes, one fewer than the normal 46. The missing one is an X chromosome.
Can a person with Turner's syndrome have children?
No. She is sterile because her ovaries are rudimentary (not fully developed), so she cannot produce eggs.
What causes Turner's syndrome?
It is caused by the absence of one X chromosome. This aneuploidy usually results from non-disjunction (failure of the sex chromosomes to separate) during formation of the egg or sperm.
Is Turner's syndrome the same as Down's syndrome?
No. Down's syndrome is trisomy 21 — an EXTRA chromosome 21. Turner's syndrome is a MISSING X chromosome (45, X0). One adds a chromosome, the other removes one.
