Klinefelter syndrome

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Klinefelter syndrome
Klinefelter syndrome defines a group of chromosomal disorders in which there is at least one extra X chromosome compared with the normal 46,XY male karyotype. The effects on physical features and on physical and cognitive development increase with the number of extra X's, and each extra X is associated with an intelligence quotient (IQ) decrease of approximately 15-16 points, with language most affected, particularly expressive language skills.


Klinefelter syndrome karyotype

Klinefelter syndrome (47,XXY, XXY syndrome or condition, XXY trisomy) affects male physical and cognitive development. The extra copy of genes on the X chromosome interferes with male sexual development and can prevent the normal function of the testis testes, reducing the levels of testosterone. The signs and symptoms vary among affected individuals.

Rare variations include 47,XXY/46,XY (mosaic syndrome) and Poly-X Klinefelter syndrome: 48,XXYY (or tetrasomy), 48,XXXY (or tetrasomy) and 49,XXXXY (or pentasomy).

First described in 1942 by Dr Harry Klinefelter.[1]

Genetic Links: genetic abnormalities | maternal age | Trisomy 21 | Trisomy 18 | Trisomy 13 | Trisomy X | trisomy mosaicism | Monosomy | Fragile X | Williams | Alagille | Philadelphia chromosome | mitochondria | VACTERL | hydatidiform mole | epigenetics | Prenatal Diagnosis | Neonatal Diagnosis | meiosis | mitosis | International Classification of Diseases | genetics

Genital System - Abnormalities

Some Recent Findings

Human X chromosome
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<pubmed limit=5>Klinefelter syndrome</pubmed>


Affects male physical and cognitive development. Small testes that do not produce as much testosterone as usual.

  • delayed or incomplete puberty, breast enlargement (gynecomastia), reduced facial and body hair, and an inability to have biological children (infertility).
  • Some affected individuals also have genital differences - undescended testes (cryptorchidism), (hypospadias), or an unusually small penis (micropenis).
  • Children may have learning disabilities and delayed speech and language development. Tend to be quiet, sensitive, and unassertive, but personality characteristics vary among affected individuals.
  • Older children and adults tend to be taller than their peers.

(Text modified from Genetics Home Reference - Klinefelter syndrome)


Cytogenetics Tests

  • Karyotyping
  • FISH-interphase

Molecular Genetics Tests

  • Detection of homozygosity
  • Deletion/duplication analysis
  • Targeted variant analysis
  • Sequence analysis of the entire coding region


  1. Klinefelter HF. Reifenstein EC. and Albright F. Syndrome characterized by gynecomastia aspermatogenes without A-Leydigism and increased excretion of follicle stimulating hormone. (1942) J Clin Endocrinol Metab. 2:615–627.



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Cite this page: Hill, M.A. (2021, March 8) Embryology Klinefelter syndrome. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Klinefelter_syndrome

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© Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G