Sex chromosome disorders: Klinefelter and Turner !

Topic

An extra X chromosome in a male (Klinefelter) or a missing one in a female (Turner) mainly affects the gonads, height and puberty.

Updated 2026-11-01

What it is

Sex chromosome aneuploidies are tolerated better than autosomal ones because extra X chromosomes are inactivated and the Y carries few genes. Even so, Klinefelter syndrome (47,XXY) is the commonest genetic cause of male hypogonadism and infertility, and Turner syndrome (45,X) is the commonest chromosomal cause of primary amenorrhoea.

Structure

Testis in Klinefelter: small, fibrosed, hyalinised seminiferous tubules with few or no germ cells and prominent clusters of Leydig cells.
Ovary in Turner: a streak of fibrous stroma without follicles, from accelerated loss of oocytes during fetal life.
The cystic hygroma of the neck in a fetus with Turner (dilated lymphatics) later leaves the webbed neck.

Step by step

Early in development, each female cell randomly inactivates one of its two X chromosomes (lyonisation), guided by XIST RNA.
The inactive X condenses and is seen as a Barr body next to the nuclear membrane.
Some genes escape inactivation, such as those of the pseudoautosomal region (for example SHOX).
So a missing X (Turner) leaves a single dose of those genes: short stature and ovarian failure.
And an extra X (Klinefelter) gives one dose too many: long legs and testicular failure.

The sex chromosome syndromes

Syndrome	Karyotype	Gonads and hormones	Body features	Associations
Klinefelter syndrome	47,XXY (mosaics too)	Hyalinised seminiferous tubules; low testosterone, high FSH and LH	Tall, long limbs, gynaecomastia, sparse body hair	Infertility, osteoporosis, higher risk of male breast cancer
Turner syndrome	45,X; mosaics; isochromosome Xq	Streak ovaries; low oestrogen, high FSH and LH	Short stature, webbed neck, broad chest, low hairline	Bicuspid valve, coarctation, horseshoe kidney, hypothyroidism
47,XYY	An extra Y	Fertility usually normal	Tall	Learning difficulties; many are never diagnosed
47,XXX	An extra X	Fertility usually normal	Tall	Learning and language difficulties

Test yourself

Think of the answer, then tap the question to see it.

What hormone profile does a man with Klinefelter syndrome have?

Hypergonadotropic hypogonadism: low testosterone with high FSH and LH because testicular feedback is lost.

Which cardiovascular defect is commonest in Turner syndrome?

Bicuspid aortic valve; coarctation of the aorta is also characteristic.

Why are X aneuploidies tolerated better than autosomal ones?

Because every X except one is inactivated; only genes that escape inactivation end up at an abnormal dose.

Which tumour threatens a Turner patient who carries Y chromosome material?

Gonadoblastoma of the dysgenetic gonad, which is why its removal is recommended.

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Klinefelter syndrome: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Turner syndrome: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026); Can changes in the number of chromosomes affect health and development?: MedlinePlus Genetics (MedlinePlus, US National Library of Medicine, 2026).

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