Encyclopedia of Biology

polygenic inheritance The interaction of several
genes on a phenotype trait. A series of genes at multiple
lociwhere each contributes a small additive effect on a
plant’s phenotype, for example, height in tobacco plants.

polygyny A mating system where one male mates
with more than one female, while each female mates
with only one male; believed to be the normal mating
system in animals.

polyhedral symbol The polyhedral symbol indicates
the geometrical arrangements of the coordinating
atoms about the CENTRAL ATOM. It consists of one or
more capital italic letters derived from common geo-
metric terms (tetrahedron, square plane, octahedron,
etc.), which denote the idealized geometry of the LIG-
ANDs around the COORDINATIONcenter, and an Arabic
numeral that is the coordination number of the central
atom. The polyhedral symbol is used as an affix,
enclosed in parentheses, and separated from the name
by a hyphen. Examples are T-4, SP-4, TBPY-5, SPY-5,
OC-6, and CU-8.

polymer A macromolecule of high relative molecular
mass composed of many similar or identical monomers
linked together in chains. Plastics are polymers.

polymerase chain reaction(PCR) A laboratory
technique used to rapidly amplify predetermined
regions of double-stranded DNA. Generally involves
the use of a heat-stable DNA polymerase.

polymorphic Refers to a phenotypic expression
occurring in a number of forms appearing within an
interbreeding population, such as fur coloration.

polymorphism Difference in DNA sequence among
individuals in a plant or animal population expressed
as two or more distinct forms of individuals in the
same population. Polymorphisms can be inherited or
environmentally created (polyphenism). Examples
include sickle cell anemia and the caste system of bees.

polymorphonuclear leukocyte(PMN; granular

leukocyte; granulocyte; inflammatory granulocyte;

polymorphonuclear cell) A subgroup of leukocytes

(white blood cells) filled with granules of toxic chemi-

cals that enable them to digest microorganisms by

phagocytosis. Examples of granulocytes are neu-

trophils, eosinophils, and basophils.

272 polygenic inheritance

original double-stranded DNA

separate strands

and anneal primers

new strands

desired

fragment

strands

3'

3'

3'

5'

5'

5'

5'

5'

3'

3'

3'

5'

new primers

primers

Polymerase chain reaction (PCR) is a fast, inexpensive technique

for making an unlimited number of copies of any piece of DNA.

Sometimes called “molecular photocopying,” PCR has had an

immense impact on biology and medicine, especially genetic

research.(Courtesy of Darryl Leja, NHGRI, National Institutes of

Health)