KS&A | Trisomy X

The following sections of this site specifically address Trisomy X:

Brief Introduction to Trisomy X

In 1956, Drs. Joe Tjio and Albert Lavan confirmed that there were 23 pairs of chromosomes in humans. Up until that time, it had been thought that there were 48 chromosomes, but using a more advanced experimental methodology, Tjio and Lavan were able to confirm that there were, in fact, only 46. (1) The discovery by Tjio and Lavan is generally considered to be the beginning of modern “cytogenetics,” which is the study of chromosomes and the diseases caused by either numerical and/or structural abnormalities in them.

Because it was thought that all men had 22 pairs of autosomes plus one X and one Y sex-determining chromosomes, they were (and still are) referred to as 46,XY; and because it was thought that all women had the same 22 pairs of automsomes plus two X chromosomes, they were (and still are) referred to as 46,XX.

Three years after Tjio and Lavan’s confirmation of 46 chromosomes in humans, in 1959, a young English researcher named Patricia Jacobs and her associate described the first chromosmal abnormality in man, the extra X chromosome in an estimated 80% of the men with Klinefelter syndrome. (2) Later than year, Dr. Jacobs identified the extra X in Trisomy X (47,XXX). (3)

Are women with an extra X chromosome at risk of developing Klinefelter syndrome?

The answer is “No.” Technically speaking, the name “Klinefelter syndrome” refers to the common set of symptoms described in a 1942 case study prepared by Dr. Harry Klinefelter following his examination of nine adult males being treated for endocrinological disorder. (4) A number of these symptoms pertain exclusively to males, whereas all Trisomy X individuals are women. Accordingy, women born with an extra X chromosome are referred to as Trisomy X, Triple X or 47,XXX.

Do 47,XXY and Trisomy X have much in common?

Although there can be broad variability of symptoms, ranging from some persons who are almost entirely asymptomatic to others who are several affected, Trisomy X females and 47,XXY males tend to share a number of common characteristics and challenges.

According to the latest estimates, the incidence level of Trisomy X is approximately 1 out of 900 live birth females. (5)

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Other Resources for Individuals and Families Dealing with Trisomy X

Information on this website is updated regularly, including based on the input of other individuals and families dealing this same condition. Accordingly, users are encouraged to check back often and to consider participating in any of the following free support services -

KS&A’s Online Discussion Groups
Contact KS&A’s (JavaScript must be enabled to view this email address)

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(1) Joe Hin Tjio and Albert Levan, “The Chromosome Number of Man,” American Journal of Obstetrics and Gynecology, 1956, 130:723-724.

(2) Patricia A. Jacobs and J. A. Strong, “A Case of Human Intersexuality Having a Possible XXY Sex-determining Mechanism,” Nature, 1959, 183: 302-303.

(3) Patricia A. Jacobs, et. al., “Evidence for the Existence of the Human “Super Female,” Lancet, September 26, 1959, 2:423-5.

(4) Harry F. Klinefelter, E. C. Reifenstein and Fuller Albright, “Syndrome Characterised by Gynecomastia, Aspermatogenesis with A-Leydigism, and Increased Excretion of Follicle Stimulating Hormone,” Journal of Clinical Endocrinology, 1942, 2: 615-627.

(5) [Cite]

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