Webster's New World Medical Dictionary

X-linked dominant
An X-linked trait that is
expressed when one copy of the gene for that trait is
present. In the case of an X-linked dominant dis-
ease, a single copy of the mutant gene on the X
chromosome can cause the disease in a female. An
example is a type of hereditary rickets called
hypophosphatemic rickets. See also 
autosomal
dominant trait; X-linked recessive.
X-linked recessive
A mode of genetic inheri-
tance characterized by a gene on the X chromosome
being expressed only when there is no different
gene present at that spot (locus) on a second X
chromosome. This means that X-linked traits are
expressed in males and in females who are homozy-
gous for the gene (they have a copy of the particu-
lar gene on each of their two X chromosomes). For
example, Duchenne muscular dystrophy (DMD) is
an X-linked recessive disorder. A boy with DMD has
the DMD gene on his sole X chromosome. Although
it is much rarer, a girl can have the DMD gene on
both her X chromosomes and have DMD. See also
autosomal recessive trait; X-linked dominant.
X-ray
High-energy radiation with waves shorter
than those of visible light. X-ray is used in low doses
to make images that help to diagnose diseases and
in high doses to treat cancer.
X-ray, AP
An X-ray picture in which the beams
pass through the patient anteroposteriorly (from
front to back).
X-ray, lateral
An X-ray picture that is taken from
the side.
X-ray, PA
An X-ray picture in which the beams
pass through the patient posteroanteriorly (from
back to front).
X-ray therapy
The use of X-ray radiation to treat
cancer. X-rays may be used inside or outside the
body, depending on the type of tumor involved. See
also 
radiation therapy.