XX males

XX male syndrome
Classification and external resources
Specialty	medical genetics
ICD-10	(Q98.3)
OMIM	278850
MeSH	D058531
Orphanet	393
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XX male syndrome (also called de la Chapelle syndrome, for Albert de la Chapelle, who characterized it in 1972) is a rare sex chromosomal disorder. Usually, it is caused by unequal crossing over between X and Y chromosomes during meiosis in the father, which results in the X chromosome containing the normally-male SRY gene. When this X combines with a normal X from the mother during fertilization, the result is an XX male. Rarer causes include mutation in an autosomal or X chromosomal gene or undetected mosaicism with a Y-bearing cell line.

This syndrome occurs in approximately four or five in 100,000 individuals, making it less common than Klinefelter syndrome.

Symptoms usually include small testes and subjects are invariably sterile. Individuals with this condition sometimes have feminine characteristics, with varying degrees of gynecomastia but with no intra-abdominal Müllerian tissue. According to research at the University of Oklahoma health science centers, most XX males are not stereotypically feminine and are typical boys and men although other reports suggest that facial hair growth is usually poor and libido is diminished, with notable exceptions.

Males typically have one X chromosome and one Y chromosome in each diploid cell of their bodies. Females typically have two X chromosomes. XX males have two X chromosomes, with one of them containing genetic material from the Y chromosome, making them phenotypically male; they are genetically female but otherwise appear to be male.

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