habitat disruption A disturbance of the physical environment in which a population lives.
hair bulb The base of a hair; contains cells that divide mitotically to produce columns of hair cells.
hair root The portion of a hair that extends from the skin's surface to the hair bulb.
hair shaft The portion of a hair that extends above the skin's surface.
half-life The time required for one-half of an original unstable radioactive element to be converted to a more stable daughter element.
halophiles A group of archaebacteria that are able to tolerate high salt concentrations.
haploid Cells that contain only one member of each homologous pair of chromosomes (haploid number = n). At fertilization, two haploid gametes fuse to form a single cell with a diploid number of chromosomes.
hardwoods Term applied to dicot trees, as opposed to softwoods, a term applied to gymnosperms.
Haversian canal The central opening of compact bone; contains nerves and blood vessels. PICTURE
heart The multicellular, chambered, muscular structure that pumps blood through the circulatory system by alternately contracting and relaxing. PICTURE
heartwood Inner rings of xylem that have become clogged with metabolic by-products and no longer transport water; visible as the inner darker areas in the cross section of a tree trunk.
helper T cells A type of lymphocyte that stimulates the production of antibodies by activating B cells when an antigen is present.
hemizygous Having one or more genes that have no allele counterparts. Usually applied to genes on the male's X chromosome (in humans).
hemoglobin A red pigment in red blood cells that can bind with oxygen and is largely responsible for the blood's oxygen-carrying capacity. Hemoglobin is composed of four polypeptide chains, two alpha (a) and two beta (b) chains.
hemophilia A human sex-linked recessive genetic disorder that results in the absence of certain blood-clotting factors, usually Factor VII. Hemophiliacs suffer from an inability to clot their blood.
hepatitis B A potentially serious viral disease that affects the liver; can be transmitted through sexual contact or through contact with infected blood.
herbaceous Term applied to a nonwoody stem/plant with minimal secondary growth.
herbivores Term pertaining to a heterotroph, usually an animal, that eats plants or algae. Herbivores function in food chains and food webs as primary consumers.
heterogametic sex The sex with two different chromosomes, such as males in humans and Drosophila.
heterotrophic Refers to organisms, such as animals, that depend on preformed organic molecules from the environment (or another organism) as a source of nutrients/energy.
heterotrophs Organisms that obtain their nutrition by breaking down organic molecules in foods; include animals and fungi.
heterozygous Having two different alleles (one dominant, one recessive) of a gene pair.
histamine A chemical released during the inþammatory response that increases capillary blood þow in the affected area, causing heat and redness.
histone proteins Proteins associated with DNA in eukaryote chromosomes.
homeobox genes Pattern genes that establish the body plan and position of organs in response to gradients of regulatory molecules.
homeostasis The ability to maintain a relatively constant internal environment.
hominid Primate group that includes humans and all fossil forms leading to man only.
hominoid Primate group that includes common ancestors of humans and apes.
homologous structures Body parts in different organisms that have similar bones and similar arrangements of muscles, blood vessels, and nerves and undergo similar embryological development, but do not necessarily serve the same function; e.g., the þipper of a whale and the forelimb of a horse.
homologues A pair of chromosomes in which one member of the pair is obtained from the organism's maternal parent and the other from the paternal parent; found in diploid cells. Also commonly referred to as homologous chromosomes. PICTURE
homozygous Having identical alleles for a given gene.
hormones Chemical substances that are produced in the endocrine glands and travel in the blood to target organs where they elicit a response.
human chorionic gonadotropin (hCG) A peptide hormone secreted by the chorion that prolongs the life of the corpus luteum and prevents the breakdown of the uterine lining.
Human Genome Project Federally funded project to determine the DNA base sequence of every gene in the human genome.
human immunodeficiency virus (HIV) The retrovirus that attacks T-cells in the human immune system, destroying the body's defenses and allowing the development of AIDS. PICTURE
Huntington disease A progressive and fatal disorder of the nervous system that develops between the ages of 30 and 50 years; caused by an expansion of a trinucleotide repeat and inherited as a dominant trait.
hydrogen bond A weak bond between two atoms (one of which is hydrogen) with partial but opposite electrical charges.
hydrophilic Water-loving. Term applied to polar molecules that can form a hydrogen bond with water.
hydrophobic Water-fearing.Term applied to nonpolar molecules that cannot bond with water.
hydrophytic leaves The leaves of plants that grow in water or under conditions of abundant moisture.
hydrosphere The part of the physical environment that consists of all the liquid and solid water at or near the Earth's surface.
hydrostatic skeleton Fluid-Þlled closed chambers that give support and shape to the body in organisms such as jellyÞsh and earthworms. No to be confused with the water-vascular system of echinoderms.
hypertension High blood pressure; blood pressure consistently above 140/90.
hypertonic A solution having a high concentration of solute.
hyphae The multinucleate or multicellular Þlaments that make up the mycelium (body) of a fungus (sing.: hypha).
hypothesis An idea that can be experimentally tested; an idea with the lowest level of confidence.
hypothalamus A region in the brain beneath the thalamus; consists of many aggregations of nerve cells and controls a variety of autonomic functions aimed at maintaining homeostasis. PICTURE
hypotonic A solution having a low concentration of solute.
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