XANES(X-ray absorption near edge structure) See
EXTENDED X-RAY ABSORPTION FINE STRUCTURE.
X chromosome The nuclei of human cells contain
22 autosomes (any chromosome that is not a sex chro-
mosome) and two sex chromosomes. In females, the
sex chromosomes are the two X chromosomes, and in
males there is one X chromosome and one Y chromo-
some.
xenobiotic A xenobiotic (Greek xenos “foreign”
bios“life”) is a compound that is foreign to a living
organism. Principal xenobiotics include drugs, carcino-
gens, and various compounds that have been intro-
duced into the environment by artificial means.
xenodiagnosis A way to test, using laboratory-bred
organisms, the diagnosis of certain parasitic diseases
(like Trypanosoma cruzi [Chagas’ disease] and
Trichinella spiralis[trichinosis] infection) when it is not
possible to identify the infecting organism.
xeric Dry and hot habitat, desertlike.
xerophytes Plants that grow under arid conditions
with low levels of soil and water and have water-
conserving features such as thick cuticles and sunken
stomatal pits.XO Sex is determined by genes, called sex-linked
genes. An XO individual has only one chromosome,
and the Y or X is missing. An XO human is viable
and develops ovaries but fails to produce steroid hor-
mones, and so the individual has no puberty and is
infertile. Those with Turner’s syndrome are missing
or have a damaged X chromosome, a condition that
affects about one in every 2,500 females. Certain
organisms, such as caddis, butterflies and moths,
birds, and some fish, have XO sex determination. A
male is produced when two X chromosomes are
present, and a female is produced when only one
X chromosome is present. Thus XY produces a
female and XX produces a male in Lepidoptera, the
heterogametic sex being the female in this select
group of organisms.X-ray absorption near edge structure(XANES)
SeeEXTENDED X-RAY ABSORPTION FINE STRUCTURE.xylem A tissue found in a plant’s vascular bundle
(above the phloem) and in the center of the plant,
where secondary growth occurs. Its purpose is to bring
water and minerals (nutrients) from the roots upward
throughout the plant.351