Habitat • The environment in which an organism lives.

Habituation (ha bich´ oo ay shun) • The simplest form of learning, in which an animal presented with a stimulus without reward or punishment eventually ceases to respond.

Hair cell • A type of mechanoreceptor in animals. Detects sound waves and other forms of motion in air or water.

Half-life • The time required for half of a sample of a radioactive isotope to decay to its stable, nonradioactive form.

Halophyte (hal´ oh fyte) [Gr. halos: salt + phyton: plant] • A plant that grows in a saline (salty) environment.

Haploid (hap´ loid) [Gr. haploeides: single] • Having a chromosome complement consisting of just one copy of each chromosome; designated 1n or n. (Contrast with diploid.)

Hardy-Weinberg equililbrium • The allele frequency at a given locus in a sexually reproducing population that is not being acted on by agents of evolution.

Haustorium (haw stor´ ee um) [L. haustus: draw up] • A specialized hypha or other structure by which fungi and some parasitic plants draw food from a host plant.

Haversian systems • Units of organization in compact bone that reflect the action of intercommunicating osteoblasts.

Heat-shock proteins • Chaperone proteins expressed in cells exposed to high temperatures or other forms of environmental stress.

Helper T cells (T_H) • T cells that participate in the activation of B cells and of other T cells; targets of the HIV-I virus, the agent of AIDS. (Contrast with cytotoxic T cells.)

Hematocrit (heme at´ o krit) [Gr. heaema: blood + krites: judge] • The proportion of 100 cc of blood that consists of red blood cells.

Hemizygous (hem´ ee zie´ gus) [Gr. hemi: half + zygotos: joined] • In a diploid organism, having only one allele for a given trait, typically the case for X-linked genes in male mammals and Z-linked genes in female birds. (Contrast with homozygous, heterozygous.)

Hemoglobin (hee´ mo glow bin) [Gr. heaema: blood + L. globus: globe] • Oxygen-transporting protein found in the red blood cells of vertebrates (and found in some invertebrates).

Hensen's node • In avian embryos, a structure at the anterior end of the primitive groove; determines the fates of cells passing over it during gastrulation.

Hepatic (heh pat´ ik) [Gr. hepar: liver] • Pertaining to the liver.

Hepatic duct • Duct that conveys bile from the liver to the gallbladder.

Herbivore (ur´ bi vore) [L. herba: plant + vorare: to devour] • An animal that eats plant tissues. (Contrast with carnivore, detritivore, omnivore.)

Heritable • Able to be inherited; in biology refers to genetically influenced traits.

Hermaphroditism (her maf´ row dite ism) [Gr. Hermes (messenger god) + Aphrodite (goddess of love)] • The coexistence of both female and male sex organs in the same organism.

Hertz (abbreviated Hz) • Cycles per second.

Hetero- [Gr.: heteros: other, different] • A prefix specifying that two or more different conditions are involved; for example, heterotroph, heterozygous.

Heterochromatin • Chromatin that retains its coiling during interphase; generally not transcribed. (Contrast with euchromatin.)

Heterochrony • A process in which the relative timing of two different developmental processes shifts independently of one another.

Heterocyst • A large, thick-walled cell in the filaments of certain cyanobacteria; performs nitrogen fixation.

Heterogeneous nuclear RNA (hnRNA) • The product of transcription of a eukaryotic gene, including transcripts of introns.

Heterokaryon • In fungi, hypha containing two genetically different nuclei.

Heteromorphic (het´ er oh more´ fik) [Gr. heteros: different + morphe: form] • having a different form or appearance, as two heteromorphic life stages of a plant. (Contrast with isomorphic.)

Heterosporous (het´ er os´ por us) • Producing two types of spores, one of which gives rise to a female megaspore and the other to a male microspore. (Contrast with homosporous.)

Heterosis • Situation in which heterozygous genotypes are superior to homozygous genotypes with respect to growth, survival, or fertility. Also called hybrid vigor.

Heterotherm • An animal that regulates its body temperature at a constant level at some times but not others, such as a hibernator.

Heterotroph (het´ er oh trof) [Gr. heteros: different + trophe: food] • An organism that requires preformed organic molecules as food. (Contrast with autotroph.)

Heterozygous (het´ er oh zie´ gus) [Gr. heteros: different + zygotos: joined] • Of a diploid organism having different alleles of a given gene on the pair of homologues carrying that gene. (Contrast with homozygous.)

Hibernation [L. hibernum: winter] • The state of inactivity of some animals during winter; marked by a drop in body temperature and metabolic rate.

Hierarchical sequencing • An approach to DNA sequencing in which markers are mapped and DNA sequences are aligned by matching overlapping sites of known sequence.

Highly repetitive DNA • Short DNA sequences present in millions of copies in the genome, next to each other (in tandem). In reassociation experiments, denatured highly repetitive DNA reanneals very quickly.

Hippocampus • A part of the forebrain that takes part in long-term memory formation.

Histamine (hiss´ tah meen) • A substance released by damaged tissue, or by mast cells in response to allergens. Histamine increases vascular permeability, leading to edema (swelling).

Histology • The study of tissues.

Histone • Any one of a group of basic proteins forming the core of a nucleosome, the structural unit of a eukaryotic chromosome. (Compare with nucleosome.)

hnRNA • See heterogeneous nuclear RNA.

Holocene • One of two epochs comprising the current geologic period, the Quaternary.

Homeobox • A 180-base-pair segment of DNA found in certain genes (called Hox genes), perhaps regulating the expression of other genes and thus controlling large-scale developmental processes.

Homeostasis (home´ ee o sta´ sis) [Gr. homos: same + stasis: position] • The maintenance of a steady state, such as a constant temperature or a stable social structure, by means of physiological or behavioral feedback responses.

Homeotherm (home´ ee o therm) [Gr. homos: same + thermos: heat] • An animal that maintains a constant body temperature by its own internal heating and cooling mechanisms. (Contrast with heterotherm, poikilotherm.)

Homeotic genes (home ee ot´ ic) • Genes that determine the developmental fate of entire segments of an animal.

Homeotic mutations • Mutations in homeotic genes that drastically alter the characteristics of a particular body segment, giving it the characteristics of other segments (as when wings grow from a Drosophila thoracic segment that should have produced legs).

Homo- [Gr. homos: same] • Prefix indicating two or more similar conditions, structures, or processes. Contrast to hetero-.

Homolog (home´ o log´) [Gr. homos: same + logos: word] • One of a pair (or larger set, of chromosomes having the same overall genetic composition and sequence. In diploid organisms, each chromosome inherited from one parent is matched by an identical (except for mutational changes) chromosome—its homolog—from the other parent.

Homology (ho mol´ o jee) [Gr. homologia: of one mind; agreement] • A similarity between two structures that is due to inheritance from a common ancestor. The structures are said to be homologous. (Contrast with analogy.)

Homoplasy (home´ uh play zee) [Gr. homos: same + plastikos: shape, mold] • The presence in several species of a trait not present in their most common ancestor. Can result from convergent evolution, reverse evolution, or parallel evolution.

Homosporous • Producing a single type of spore that gives rise to a single type of gametophyte, bearing both female and male reproductive organs. (Contrast with heterosporous.)

Homozygous (home´ oh zie´ gus) [Gr. homos: same + zygotos: joined] • In a diploid organism, having identical alleles of a given gene on both homologous chromosomes. An individual may be a homozygote with respect to one gene and a heterozygote with respect to another. (Contrast with heterozygous.)

Hormone (hore´ mone) [Gr. hormon: to excite, stimulate] • A substance produced in minute amount at one site in a multicellular organism and transported to another site where it acts on target cells.

Host • An organism that harbors a parasite or symbiont and provides it with nourishment.

Hot desert • Biome that is found in two belts, centered around 30 degrees north and 30 degrees south latitudes and receives most of its rainfall in summer, but also receives winter rains from storms that form over the mid-latitude oceans.

Hox genes • See homeobox.

Humoral immune response • The part of the immune system mediated by B cells that produce circulating antibodies active against extracellular bacterial and viral infections.

Humus (hew´ muss) • The partly decomposed remains of plants and animals on the surface of a soil.

Hyaluronidase ((high´ uh loo ron´ uh dase) • An enzyme that digests proteoglycans. In sperm cells, it digests the coatings surrounding an egg so the sperm can enter.

Hybrid (high´ brid) [L. hybrida: mongrel] • The offspring of genetically dissimilar parents. In molecular biology, a double helix formed of nucleic acids from different sources.

Hybrid vigor • See heterosis.

Hybrid zone • A narrow zone where two populations interbreed, producing hybrid individuals.

Hybridoma • A cell produced by the fusion of an antibody-producing cell with a myeloma cell; it produces monoclonal antibodies.

Hydrocarbon • A compound containing only carbon and hydrogen atoms.

Hydrogen bond • A weak electrostatic bond which arises from the attraction between the slight positive charge on a hydrogen atom and a slight negative charge on a nearby oxygen or nitrogen atom.

Hydrological cycle • The movement of water from the oceans to the atmosphere, to the soil, and back to the oceans.

Hydrolysis (high drol´ uh sis) • [Gr. hydro: water + lysis: break apart] • A chemical reaction that breaks a bond by inserting the components of water: AB + H₂O -> AH + BOH.

Hydrophilic (high dro fill´ ik) [Gr. hydro: water + philia: love] • Having an affinity for water. (Contrast with hydrophobic.)

Hydrophobic (high dro foe´ bik) [Gr. hydro: water + phobia: fear] • Having no affinity for water. Uncharged and nonpolar groups of atoms are hydrophobic, for example fats and side chain of the amino acid phenylalanine. (Contrast with hydrophilic.)

Hydrostatic pressure • Pressure generated by compression of liquid in a confined space. Generated in plants, fungi, and some protists with cell walls by the osmotic uptake of water. Generated in animals with closed circulatory systems by the beating of a heart.

Hydrostatic skeleton • The incompressible internal liquids of some animals that transfer forces from one part of the body to another when acted upon by the surrounding muscles.

Hydroxyl group • The –OH group found on alcohols and sugars.

Hyper- [Gk. hyper: above, over] • Prefix indicating above, higher, more.

Hyperpolarization • A change in the resting potential of a membrane so the inside of a cell becomes more electronegative. (Contrast with depolarization.)

Hypersensitive response • A defensive response of plants to microbial infection; it results in a "dead spot."

Hypertension • High blood pressure.

Hypertonic • Having a greater solute concentration. Said of one solution compared to another. (Contrast with hypotonic, isotonic.)

Hypha (high´ fuh) (plural: hyphae) [Gr. hyphe: web] • In the fungi and oomycetes, any single filament.

Hypo- [Gk. hypo: beneath, under] • Prefix indicating underneath, below, less.

Hypoblast • The lower tissue portion of the avian blastula which is joined to the epiblast at the margins of the blastodisc.

Hypocotyl [Gk. hypo: beneath + kotyledon: hollow space] • That part of the embryonic or seedling plant shoot that is below the cotyledons.

Hypothalamus • The part of the brain lying below the thalamus; it coordinates water balance, reproduction, temperature regulation, and metabolism.

Hypothesis • A tentative answer to a question, from which testable predictions can be generated. (Contrast with theory.)

Hypothesis-prediction method • A method of science in which hypotheses are generated, predictions are made from them, and experiments and observations are performed to test the predictions.

Hypotonic • Having a lesser solute concentration. Said of one solution in comparing it to another. (Contrast with hypertonic, isotonic.)