the private sector; and address the ethical, legal, and
social issues (ELSI) that may arise from the project.
See alsoGENE.
humoral immunity A form of immune reaction that
attacks bacteria and viruses found in body fluids using
antibodies synthesized by the B lymphocytes that circu-
late in blood plasma and lymph. These fluids were once
called “humors.”
Huxley, T. H. A19th-century evolutionist who dis-
covered the unbroken horse lineage from the Eocene to
the Holocene (recent) epochs, perhaps the longest and
most complete evolutionary sequence in the fossil
record.
hyaline A clear or transparent structure such as a
wing of a dragonfly (e.g., Aeshna canadensisand Sym-
petrum vicinum), or an amorphous texture due to
accumulation of intra- or extracellular material.
hybrid The offspring produced by genetically distinct
different parents. Mating can be within species
(intraspecific) as well as between species (interspecific).
hybridization Producing hybrids from interbreeding
two species. In genetics, it is the annealing of two com-
plementary strands of DNA, or an RNA strand to a
complementary DNA strand.
hybridoma The fusion of two different cells to cre-
ate a hybrid cell that secretes a single specific antibody,
e.g., the fusion of a spleen cell and a cancer cell, or a T
lymphocyte with a lymphoma cell.
hybrid vigor Increased vitality or success of a hybrid
over its inbred parents.
hybrid zone Ageographical territory where previ-
ously isolated or genetically distinct populations make
contact and form hybrids. Where two geographical
races of a single species overlap, hybrids and intermedi-
ates can outnumber the pure forms in the overlap
region. What determines whether there will be two
species or one is the balance between gene flow and
selection against hybrids.hydration Addition of water or the elements of
water (i.e., H and OH) to a molecular entity. The term
is also used in a more restricted sense for the process: A
(gas) ÔA (aqueous solution).
See alsoAQUATIONand SOLVATION.hydrocarbon A compound made of only carbon and
hydrogen.hydrocephalus A condition in which the head
becomes enlarged and expanded beyond normal size
due to congenital or other causes.hydrogenase An ENZYME, dihydrogen acceptor OXI-
DOREDUCTASE, that catalyzes the formation or oxida-
tion of H 2. Hydrogenases are of various types. One
class ([Fe]-hydrogenases) contains only IRON–SULFUR
CLUSTERs. The other major class ([NiFe]-hydrogenases)
has a nickel-containing center and iron–sulfur clusters;
a variation of the latter type ([NiFeSe]-hydrogenases)
contains selenocysteine.hydrogen bond Aform of association between an
electronegative atom and a hydrogen atom attached to
a second, relatively electronegative atom. It is best
considered as an electrostatic interaction, heightened
by the small size of hydrogen, that permits proximity
of the interacting dipoles or charges. Both electronega-
tive atoms are usually (but not necessarily) from the
first row of the periodic table, e.g., N, O, or F. Hydro-
gen bonds can be intermolecular or intramolecular.
With a few exceptions, usually involving fluorine, the
associated energies are less than 20–25 kJ mol–1(5–6
kcal mol–1).
A type of bond formed when the partially positive
hydrogen atom of a polar covalent bond in one168 humoral immunity