Life Science Dictionary H


Habitat: The place or type of site where an organism naturally lives and grows.
Habituation: A very simple type of learning that involves a loss of responsiveness to stimuli that convey little or no information.
Hair cell: A type of mechanoreceptor that detects sound waves and other forms of movement in air or water.
Haldane’s rule: When in the offspring of two different animal taxa, one sex is absent, rare, or sterile, that sex is the sex with heterozygous sex chromosomes.
Half-life: The number of years that takes for 50% of a sample of an isotope to decay.
Halophilic: Describes an organism with a preference for growth in high-salt environments.
Haltere: Sense organ found in Dipteria on the second thoracic segment, evolutionary derived by modification of the wing. They are used to help balance during flight.
Handicap: A trait that signals a male’s genetic quality. It’s association with good genes is maintained because it is less costly to males of higher quality.
Haplodiploid: A system of sex determination in which fertilized eggs develop as diploid females and unfertilized eggs develop as haploid males.
Haploid: Carrying one copy of each chromosome.
Haplotype: A particular combination of alleles in a haploid; that is a haploid genotype.
Hardy-Weinberg proportions: The frequencies of diploid genotypes produced after random mating.
Haustorium: In certain symbiotic fungi, specialized hyphae that can penetrate the tissues of host organisms.
Heart: A muscular pump that uses metabolic energy to elevate hydrostatic pressure of the blood. Blood the flows down a pressure gradient through blood vessels that eventually return blood to the heart.
Heart attack: The death of cardiac muscle tissue resulting in prolong blockage of one or more coronary arteries.
Heart murmur: A hissing sound that occurs when blood squirts backwards through a leaky valve in the heart.
Heart rate: The rate of heart contraction.
Heartwood: Older layer of secondary xylem, closer to the center of a stem or root, that no longer transport xylem sap.
Heat: The total amount of kinetic energy due to molecular motion in a body of matter. Heat is energy in its most random form.
Heat of vaporization: The quantity of heat a liquid must absorb for 1 g of ti to be converted from the liquid to the gaseous state.
Heat-shock protein: A protein that helps protect other proteins during heat stress. Heat-shock proteins are found in plants, animals, and microorganisms.
Heavy chain: One or the two types of polypeptide chains that make up an antibody molecule and B cell receptor; consisting of a variable region, which contributes to the antigen-binding site, and a constant region.
Helicase: An enzyme that untwists the double helix of DNA at the replication fork.
Helper T cell: A type of T cell that, when activated, secrets cytokines that promote the response of B cells (humoral response) and cytotoxic T cells (cell-mediated response) to antigens.
Hemichordate: Member of a diverse phylum of marine animals including the acorn worm and pterobranchs (phylum Hemichordata).
Hemimetabolous: Developing directly through a series of nymphal stages with a similar morphology to the adult (e.g. as in grasshoppers and bugs). It contrasts with holometabolous.
Hemocyanin:  A type of respiratory pigment that uses copper as its oxygen-binding component. Hemocyanin is found in the hemolymph of arthropods and many molluscs.
Hemoglobin: An iron containing protein in red blood cells that reversibly binds oxygen.
Hemolymph: In invertebrates with an open circulatory system, the body fluid that bathes tissues.
Hemophilia: A human genetic disease caused bu a sex-linked recessive allele, characterized by excessive bleeding  following injury.
Hepatic portal vein: A large circulatory channel that conveys nutrient-laden blood from the small intestine to the liver, which regulates the blood’s nutrient content.
Herbaceous: Referring to nonwoody plants.
Herbivore: Feeding on plants. A heterotrophic animal that eats plants.
Heredity: The transmission of traits from one generation to the next.
Heritability: The fraction of phenotypic variance that is inherited.
Hermaphrodite: An individual that produces both male and female gametes.
Heterochromatin: Nontranscribed eukaryotic chromatin that is so highly compacted that is visible with a light microscope during interphase.
Heterochrony: The change in the relative timing or duration of events during development achieve by altering the relative onset or ending of particular developmental process.
Heterocyte: A specialized cell that engages in nitrogen fixation in some filamentous cyanobacteria. Also called heterocyst.
Heterogametic: The sex that carries distinct sex chromosomes. For example, in mammals males are the heterogametic sex.
Heterokaryon: A fungal mycelium formed by the fusion of two hyphae that have genetically different nuclei.
Heterokont: One of the kingdoms of eukaryotes. It includes a diverse collection of single-celled species including diatoms.
Heteromorphic: Referring to a condition in the life cycle of all living plants and certain algae in which the sporophyte and gametophyte differ in morphology.
Heterosis: The increase in fitness seen in a cross between different populations.
Heterosporous: A term referring to a plant species that have two kinds of spores: microspores that develop into male gametophyte and megaspores that develop into a female gametophyte.
Heterostyly: A polymorphism for distinct arrangements of anther and stigma.
Heterotroph: An organism that obtains organic food molecules by eating other organisms or their by-products.
Heterozygote: A diploid individual that carries two different alleles at a locus.
Heterozygote advantage: Greater reproductive success of heterozygous individuals compared to homozygotes; tends to preserve variation in gene pools.
Hexapod: An insect or closely related wingless six-legged arthropod.
Hibernation: The act to pass the winter in a torpid or resting state. A physiological state that allows survival during long periods of cold temperatures and reduced food supplies in which the metabolism decreases, the heart and respiratory system slows down, and body temperature is maintained at a lower level than normal.
Hierarchical classification: A system of classifying organisms in a nested series of levels: domain, kingdom, phylum, class, order, family, genus, species.
High-density lipoprotein (HDL): A cholesterol carrying particle in the blood, made up of cholesterol and other lipids surrounded by a single layer of phospholipids in which proteins are embedded.
Hill-Robertson effect: The interference between selection at linked loci.
Hind brain: One of three ancestral and embryonic regions of the vertebrate brain, develops into the medulla oblongata, pons, and cerebellum.
Histamine: A substance released by mast cells that cause blood vessels to dilate and become more permeable during an inflammatory response.
Histone: A small protein with a high proportion of positively charged amino acids that binds to the negatively charged DNA and plays a key role in its chromatin structure.
Histone acetylation: The attachment of acetyl groups to certain amino acids of histone proteins.
Hitchhiking: The increase in a neutral allele that happens to be associated with a selectively favorable allele at another locus. It is sometimes used more broadly to refer to any kind of indirect selection.
HIV: Human immunodeficiency virus.
HLA: Human leukocyte antigen.
Holdfast: A rootlike structure that anchors a seaweed.
Holliday junction: The cross-linking structure formed by crossing over between two DNA double helices.
Holoblastic cleavage: A type of cleavage in which there is complete division of the egg.
Holometabolous: Metamorphosis through a pupal stage (as in butterflies, and beetles). It contrasts with hemimetabolous.
Homeobox: A sequence, approximately 180 nucleotides long that is translated into a DNA binding domain called the homeodomain. This sequence in found in many transcription factors that play a role in pattern formation and cell differentiation in animals. Related sequences occur in plants and prokaryotes.
Homeodomain: A sequence, approximately 60 amino acids long, that is encoded by a homeobox DNA sequence.
Homeostasis: The steady-state physiological condition of the body.
Homeotic: Describe a class of mutations that transform one part of an organism into another part. For example, a particular homeotic mutation in Drosophila transform the antennae into legs.
Homeotic gene: Any of the genes that control the overall body plan of animals and plants by controlling the developmental fate of groups of cells.
Hominid: Member of the great apes (family Hominidae) which now include human, gorilla, orangutan, and chimpanzee.
Hominin: All taxa closer to humans than to chimpanzee. Apart from ourselves; all these taxa are now extinct.
Hominoid: A term that refers to great apes and humans.
Homologous chromosomes:  Chromosomes pairs of the same length, centromere position, and staining pattern that possess genes for the same characters at corresponding loci. One homologous chromosome is inherited from the organism’s father, the other from the mother.
Homologous recombination: The process by which two pieces of DNA, identical or nearly identical in sequence (e.g. two copies of a chromosome), align and exchange a portion of DNA.
Homologous structures: Structures in different species that are similar because of common ancestry.
Homology: A similarity attributable to its derivation from the same ancestral feature. In genetic studies, it refers to genes that are present in the same locus in the genome. In phenotypic studies, it refers to character traits or states that are present in a set of species and their common ancestor.
Homoplasy: A similarity of traits that is not due to homology but instead to convergence or parallel evolution.
Homosporous: A term referring to a plant species that has a single kind of spores, which typically develops into a bisexual gametophyte.
Homozygote: A diploid individual that carries the same allele at a genetic locus.
Homunculus: A “little man” that was supposedly introduced into a fertilized egg by the sperm and that guided its development.
Horizon: A distinct layer of soil, such as top soil.
Horizontal cell: A neuron of the retina that helps integrate information before it is sent to the brain
Horizontal transmission: Transmission of genetic information between different individuals other than from parent to offspring.
Hormone: In multicellular organisms, one of many types of circulating chemical signals that are formed in specialized cells, travel in body fluids, and act on specific target cells to change their functioning.
Hornwort: A small, herbaceous nonvascular plant that is a member of the phylum Anthocerophyta.
Host: The larger participant in a symbiotic relationship, serving as home and feeding ground to the symbiont.
Host races: Genetically distinct populations that specialized on different hosts.
Host range: The limited range of host cells that each type of virus cam infect and parasitize.
Human chorionic gonadotropin (HCG):  Hormone secreted by the chorion that maintains the corpus luteum of the ovary during the first three months of pregnancy.
Human genome project (HGP): An international collaborative effort to map and sequence the DNA of the entire human genome.
Humoral immune response:  The branch of acquired immunity that involves the activation of B cells and that leads to the production of antibodies, which defends against bacteria and viruses in body fluids.
Humus: Decomposing organic material found in topsoil.
Huntington’s disease: A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10-20 years after the onset of symptoms.
Hybrid rescue allele: An allele that alleviates hybrid sterility or inviability.
Hybrid zone: A narrow region in which genetically distinct populations meet, mate, and hybridize.
Hybridization:  In genetics, the mating or crossing of two true-breeding varieties.
Hydration shell: The sphere of water molecules around each dissolved ion.
Hydrogen bond:  A type of weak chemical bond formed when the slightly positive hydrogen ion of a polar covalent bond in one molecule is attracted to the slightly negative atom of a polar covalent bond in another molecule.
Hydrogen ion: A single proton with a charge of +1. The dissociation of a water molecule (H2O) leads to the generation of a hydroxide ion OHand a hydrogen ion H+
Hydrogenosome: An organelle in some eukaryotes that produces hydrogen gas and ATP. It is possibly derived from mitochondria.
Hydrolysis: A chemical process that lyses or splits molecules by the addition of water.
Hydrophilic: A molecule or portion of a molecule that readily dissolves in water via the formation of hydrogen bonds.
Hydrophobic: A molecule or portion of a molecule that does not readily dissolves in water.
Hydrophobic core: A portion of a protein that avoids dissolution in water and is composed of a set of hydrophobic amino acids.
Hydrophobic interaction: A type of weak chemical bond when molecules that do not mix with water coalesce to exude the water.
Hydroponic culture: A method in which plants are grown without soil by using mineral solutions.
Hydrostatic skeleton: A skeletal system composed of fluid held under pressure in a closed body compartment; the main skeleton of most cnidarians, flatworms, and annelids.
Hydroxide ion: A water molecule that has lost a proton OH.
Hydroxyl group: A functional group consisting of a hydrogen atom joined to an oxygen atom by a polar covalent bond. Molecules possessing this group are called alcohols.
Hypercycle: Cooperation between a set of replicating molecules (A, B, C, …. Z), in which A aids replication of B, B aids C, … and Z aids A. It was proposed as a way for early life to replicated genetic information despite high mutation rates.
Hyperpolarization:  An electrical state in which the inside of the cell is more negative relative to the outside than at the resting membrane potential.
Hypersensitive response (HR): A plant’s localized defense response to a pathogen.
Hypertension: Chronically high blood pressure within the arteries.
Hyperthermophile: An organism that thrives at temperatures above 80 degrees centigrade.
Hypertonic: In comparing two solutions, referring to the one with a greater solute concentration.
Hypha: A filament that collectively makes up the body of a fungus. plural: hyphae.
Hypocotyl: In an angiosperm embryo, the embryonic axis below the point of attachment of the cotyledon(s) and above the radicle.
Hypothalamus: The ventral part of the vertebrate forebrain; functions in maintaining homeostasis, especially in coordinating the endocrine and nervous systems; secrets hormones of the posterior pituitary and releasing factor that regulate the anterior pituitary.
Hypothesis: A tentative answer to a well-framed question.
Hypotonic: In comparing two solutions, referring to the one with a lower solute concentration.