XYY syndrome

Overview
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.

Etiology
47,XYY is not inherited, but usually occurs as a random event during the formation of sperm cells. An error in chromosome separation during metaphase II (of meiosis II) called nondisjunction can result in sperm cells with an extra copy of the Y chromosome. If one of these atypical sperm cells contributes to the genetic makeup of a child, the child will have an extra Y chromosome in each of the body's cells.

In some cases, the addition of an extra Y chromosome results from nondisjunction during cell division during a post-zygotic mitosis in early embryonic development. This can produce 46,XY/47,XYY mosaics.

Incidence
About 1 in 1,000 boys are born with a 47,XYY karyotype. The incidence of 47,XYY is not affected by advanced paternal or maternal age.

First case
The first published report of a man with a 47,XYY karyotype was by Avery A. Sandberg and colleagues at Roswell Park Memorial Institute in Buffalo, New York in 1961. It was an incidental finding in a normal 44-year-old, 6 ft. [183 cm] tall man of average intelligence who was karyotyped because he had a daughter with Down syndrome.

47,XYY was the last of the common sex chromosome aneuploidies to be discovered, two years after the discoveries of 47,XXY, 45,X, and 47,XXX in 1959. Even the much less common 48,XXYY had been discovered in 1960, a year before 47,XYY. Screening for these X chromosome aneuploidies was possible by noting the presence or absence of "female" sex chromatin bodies (Barr bodies) in the nuclei of interphase cells in buccal smears, a technique developed a decade before the first reported sex chromosome aneuploidy.

An analogous technique to screen for Y chromosome aneuploidies by noting supernumerary "male" sex chromatin bodies was not developed until 1970, a decade after the first reported sex chromosome aneuploidy. In December 1969, Lore Zech at the Karolinska Institute in Stockholm first reported intense fluorescence of the AT-rich distal half of the long arm of the Y chromosome in the nuclei of metaphase cells treated with quinacrine mustard. Four months later, in April 1970, Peter L. Pearson and Martin Bobrow at the MRC Population Genetics Unit in Oxford and Canino G. Vosa at the University of Oxford reported fluorescent "male" sex chromatin bodies in the nuclei of interphase cells in buccal smears treated with quinacrine dihdyrochloride.

Physical traits
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. 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.

Testosterone levels (prenatally and postnatally) are normal in 47,XYY males. 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.

Behavioral characteristics
47,XYY boys have an increased risk of learning difficulties (in up to 50%) and delayed speech and language skills. 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.

As with 47,XXY boys and 47,XXX girls, IQ scores of 47,XYY boys average 10–15 points below their siblings. 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. 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.

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. Aggression is not seen more frequently in 47,XYY males.