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{...
The most common cause of
sex chromosome aneuploidies is
nondisjunction, which can occur during meiosis or during the early stages of postzygotic development. The loss or gain of genetic material can affect all daughter cells or it may be partial, leading to tissue mosaicism. In both typical and atypical sex chromosome
karyotypes, there is random inactivation of all but one
X chromosome.
...
Sex chromosome aneuploidies are associated with several NDDs. Mild to moderate decreases in general cognitive abilities have been reported in XO, XXX,
XXY, XYY, and XXYY populations (Lee et al., 2012; Printzlau et al., 2017). Social-communication deficits have been identified in all
sex chromosome aneuploidies, with 20% of XO girls (Wolstencroft et al., 2018), 10% of XXY boys, 38% of XYY boys, and 52% of XXYY boys meeting criteria for an ASD (Tartaglia et al., 2017).
ADHD is also relatively common and occurs in 24% of XO girls (Russell et al., 2006), 52% of XXX girls, 36% of XXY boys, 76% of XYY boys, and 72% of XXYY boys (Tartaglia et al., 2012). These observations show an important dosage-dependent role for both the
X and Y chromosomes in NDDs and suggest that boys may be more vulnerable to NDDs partly because of their
Y chromosome or their lack of a second
X chromosome.
...}