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XYY syndrome is an aneuploidy of the sex chromosomes in which a human male receives an extra Y chromosome, producing a 47,XYY karyotype.
Some medical geneticists question whether the term “syndrome” is appropriate for this condition because its phenotype is normal and the vast majority (an estimated 97% in the UK) of 47,XYY males do not know their karyotype.[1][2]
Most often, the extra Y chromosome causes no unusual physical features or medical problems. 47,XYY boys have an increased growth velocity during earliest childhood, with an average final height approximately 7 cm above expected final height.[3] Severe acne was noted in a very few early case reports, but dermatologists specializing in acne now doubt the existence of a relationship with 47,XYY.[4]
Testosterone levels (prenatally and postnatally) are normal in 47,XYY males.[5] Most 47,XYY males have normal sexual development and usually have normal fertility. Since XYY is not characterized by distinct physical features, the condition is usually detected only during genetic analysis for another reason.
47,XYY boys have an increased risk of learning difficulties (in up to 50%) and delayed speech and language skills.[1][6][7][8][9][10][11][12] In contrast, a national survey of US children conducted in 2004 for the CDC found that 10% of 46,XY boys had a learning disability.[13]
As with 47,XXY boys and 47,XXX girls, IQ scores of 47,XYY boys average 10–15 points below their siblings.[6][8][9][11] It is important to realize that this amount of variation — an average difference of 12 IQ points — occurs naturally between children in the same family.[6] In 14 prenatally diagnosed 47,XYY boys from high socioeconomic status families, IQ scores available for 6 boys ranged from 100–147 with a mean of 120. For 11 boys with siblings, in 9 instances their siblings were stronger academically, but in one case they were performing equal to and in another case superior to their brothers and sisters.[14]
Developmental delays and behavioral problems are also possible, but these characteristics vary widely among affected boys and men, are not unique to 47,XYY and are managed no differently than in 46,XY males.[8][12] Aggression is not seen more frequently in 47,XYY males.[1][6][8][9][10]
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