Webster's New World Medical Dictionary

polycythemia vera
A disorder of the bone mar-
row that leads to overproduction of all three blood
cell lines: white blood cells, red blood cells, and
platelets. Its cause is unknown. Abbreviated PV. PV
progresses slowly but may evolve into acute
leukemia or myelofibrosis, in which the marrow is
replaced by scar tissue. For a diagnosis of PV, there
must be polycythemia. See also 
polycythemia.
polydactyly
More than the normal number of
fingers or toes. The opposite of polydactyly is oligo-
dactyly. See also 
hexadactyly.
polydipsia
Constant, excessive drinking as a
result of thirst. Polydipsia occurs in untreated or
poorly controlled diabetes mellitus.
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polydipsia
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polygenes
Many genes. For example, eye color is
polygenically controlled because many genes are
involved in the determination of eye color.
polygenic disease
A genetic disorder that is
caused by the combined action of more than one
gene. Examples of polygenic conditions include
hypertension, coronary heart disease, and diabetes.
Because such disorders depend on the simultaneous
presence of several genes, they are not inherited as
simply as are single-gene diseases.
polyhydramnios
Too much amniotic fluid. The
opposite of polyhydramnios is oligohydramnios.
polymerase, DNA
See 
DNA polymerase.
polymerase, RNA
See 
RNA polymerase.
polymerase chain reaction
A key technique in
molecular genetics that permits the analysis of any
short sequence of DNA or RNA without having to
clone it. Abbreviated PCR. PCR is used to amplify
selected sections of DNA in only a few hours. The
PCR technique has innumerable uses, from diag-
nosing genetic diseases to DNA fingerprinting. PCR
has become an essential tool for biologists, foren-
sics labs, and scientists who want to study genetic
material.