ments (folding, twisting, coiling) of polypeptide chains
in a protein. They can be in the form of a helix, ran-
dom coil, or pleated sheet; linked by hydrogen bonds in
the peptide backbone; sheets are formed when the
polypeptide chains fold back and forth, or when two
parts are parallel to each other and bonded; also called
the beta pleated sheet.
pleiotropy The ability of a single gene to affect many
phenotypic traits. Jonathan Hodgkin from the MRC
Laboratory of Molecular Biology, in Cambridge, Unit-
ed Kingdom, has characterized several types of
pleiotropy:
Artefactual Adjacent but functionally unrelated genes
affected by the same mutation, e.g., claret.
Secondary Simple primary biochemical disorder lead-
ing to complex final phenotype, e.g., phenylke-
tonurea.
Adoptive One gene product used for quite different
chemical purposes in different tissues, e.g., e-crys-
tallin.
Parsimonious One gene product used for identical
chemical purposes in multiple pathways, e.g., gpb-1.
Opportunistic One gene product playing a secondary
role in addition to its main function, e.g., sisB/
AS-C.
Combinatorial One gene product employed in vari-
ous ways, and with distinct properties, depending on
its different protein partners, e.g., unc-86.
Unifying One gene, or cluster of adjacent genes,
encoding multiple chemical activities that support a
common biological function, e.g., cha-1 unc-17.
plesiomorphic character The ancestral character of
a homologue; the descendant character is termed the
apomorphic character. Two characters in two taxa are
homologues if they are the same as the character that is
found in the ancestry of the two taxa, or if they have
characters that have an ancestor/descendant relation-
ship described as preexisting or novel (plesiomorphic
and apomorphic character).
All taxa are mixtures of ancestral and derived char-
acters. Ancestral character states are those similar to
the remote ancestor, while derived characters are those
that have undergone recent change. Taxa can show
either the ancestral (or plesiomorphic) character state
or the derived (apomorphic) character state for a par-
ticular character.pluripotent stem cell(stem cell) Primitive blood
cells found in the bone marrow, circulating blood-
stream, and umbilical cord that are capable of repro-
ducing and differentiating to make all varieties of
mature blood cells (white blood cells, red blood cells,
and platelets). This means that all blood cells originate
from this single type of cell. Pluripotent stem cells are
usually referred to as stem cells.pneumatophore(breathing roots) A specialized root
structure that has numerous pores or lenticels over its
surface, allowing gas exchange, and that grows up into
the air, e.g., mangroves.pocosin A swamp on the coastal plain of the south-
eastern United States.poikilotherm An organism (e.g., fish or reptile)
whose body temperature varies or fluctuates with the
temperature of its surroundings; an ectotherm.point mutation When the base sequence of a codon
is permanently changed. Four types exist:Missense A change in base sequence converts a codon
for one amino acid to a codon for a different amino
acid.
Nonsense A codon for a specific amino acid is
converted to a chain-terminating codon.
Silent Conversion of a codon for an amino acid to
another codon that specifies the same amino acid.
Frameshift Anucleotide is deleted or added to the
coding portion of a gene.poison plants(poison ivy; poison oak; poison sumac)
The poison ivy plant is known as Toxicodendron radi-
cansin the eastern United States and T. rydbergiiin
the midwestern United States. Historically it has been270 pleiotropy