global parameters representing hydrophobic, electron-
ic, steric, and other effects using multiple regression
correlation methodology.
haploid cell Acell containing one set of chromo-
somes. Sperm and egg cells are haploid.
haplozoans Extinct echinoderms of the Cambrian
era that include two classes, Cycloidea and Cyamoidea.
Cyamoids have bilateral symmetry, and cycloids are
dome-shaped with radial symmetry. They are quite
uncharacteristic of echinoderms, although they have a
skeletal structure that relates them. Otherwise, they
have uncertain affinities to all other echinoderms and
may provide a link between echinoderms and fish.
hapten A molecule (usually a small organic molecule)
that can be bound to an ANTIGENicdeterminant/epi-
tope. Usually they are too small to give a response of
their own. They become antigenic if they are coupled to
a suitable macromolecule, such as a protein.
hapto The hapto symbol, η(Greek eta), with numeri-
cal superscript, provides a topological description for the
bonding of hydrocarbons or other π-electron systems to
metals by indicating the connectivity between the LIGAND
and the CENTRAL ATOM. The symbol is prefixed to
the ligand name or to that portion of the ligand name
most appropriate. The superscript numerical index on
the right indicates the number of COORDINATINGatoms
in the ligand that bind to the metal. Examples:
[PtCl 2 (C 2 H 4 )(NH 3 )] amminedichloro
(η^2 -ethene)platinum[Fe(η^5 -C 5 H 5 ) 2 ] bis (η^5 -cyclopentadienyl)iron
(ferrocene)See alsoCOORDINATION.hard acid ALEWIS ACIDwith an acceptor center of
low polarizability. It preferentially associates with
HARD BASEs rather than with soft bases, in a qualitative
sense (sometimes called HSAB rule). Conversely, a soft
acid possesses an acceptor center of high polarizability
and exhibits the reverse preference for a partner for
COORDINATION.hard base ALEWIS BASEwith a donor center of low
polarizability; the converse applies to soft bases.
See alsoHARD ACID.hard drug Anonmetabolizable compound, charac-
terized either by high lipid solubility and accumula-
tion in adipose tissues and organelles, or by high
water solubility. In the lay press, the term hard drug
refers to a powerful DRUGof abuse such as cocaine or
heroin.Hardy-Weinberg theorem A mathematical theorem
that describes a population that is not evolving under
certain conditions. First proposed in 1908 by G. H.
Hardy, an English mathematician, and B. W. Weinberg,
a German physician, although both were working inde-
pendently of each other. The theory describes the fre-
quencies of various genotypes in a population and the
ability to predict how the frequencies will change in
future generations based on certain assumptions not
being violated, such as the absence of mutation, the
absence of migration or selection, random mating, and
an infinite population size. In other words, in the
absence of any of these factors, the allele frequencies
and the frequencies of the different genotypes will
remain the same from one generation to the next, and
genetic recombination due to sexual reproduction will
not result in any changes in allele frequencies or geno-
type frequencies.harelip(cleft lip) A congenital (birth) deformity
caused by a failure of developing tissues to fuse; a cleft
in the upper lip, found on one or both sides of the mid-
line. Often associated with a cleft palate.haustorium A specialized structure of a parasitic
plant or entophyte, an organism that lives at least part
of its life cycle within a host plant, that penetrates the
living host and absorbs nutrients. In parasitic fungi, it156 haploid cell