Webster's New World Medical Dictionary

XX
The sex chromosome complement of a nor-
mal human female. See also 
X chromosome.
XXX syndrome
A chromosome condition that is
present in 1 in 1,000 females, due to the presence
of three X chromosomes rather than the usual two.
The condition is associated with increased height
but no malformations. Intelligence ranges from
above normal to mild mental retardation. Also known
as triple X and triplo-X.
XY
The most frequent sex chromosome comple-
ment in human males. See also 
X chromosome; Y
chromosome.
xerophagia
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xylitol
A sweetener that is found in plants and
used as a substitute for sugar. Xylitol is called a
nutritive sweetener because it provides calories, just
like sugar. However, it is less likely than sugar to
contribute to dental caries.
XYY syndrome
A chromosomal disorder that
affects about 1 in 1,000 males caused by the pres-
ence of an extra Y chromosome. Symptoms may
include increased height, speech delays, learning
disabilities, mild to moderate mental retardation,
and behavioral disturbances. Also known as
polysomy Y syndrome.
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XYY syndrome
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Y 1
In chemistry, the symbol for the element
yttrium, an extremely rare metal that has been used
in certain nuclear medicine scans.