| habitat disruption |
| A disturbance of the physical environment in which a population lives. |
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| hair bulb |
| The base of a hair; contains cells that divide mitotically to produce columns of hair cells. |
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| hair root |
| The portion of a hair that extends from the skin's surface to the hair bulb. |
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| hair shaft |
| The portion of a hair that extends above the skin's surface. |
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| half-life |
| The time required for one-half of an original unstable radioactive element to be converted to a more stable daughter element. |
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| halophiles |
| A group of archaebacteria that are able to tolerate high salt concentrations. |
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| haploid |
| The number of chromosomes in a sperm or egg cell, half the diploid number.
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| haploinsufficiency |
| A situation in which the protein produced by a single copy of an otherwise normal gene is not sufficient to assure normal function.
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| Haversian canal |
| The central opening of compact bone; contains nerves and blood vessels. |
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| heart |
| The multicellular, chambered, muscular structure that pumps blood through the circulatory system by alternately contracting and relaxing. |
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| 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. |
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| helper T cells |
| A type of lymphocyte that stimulates the production of antibodies by activating B cells when an antigen is present. |
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| hematopoietic stem cell |
| An unspecialized precursor cell that will develop into a mature blood cell.
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| hemizygous |
| Having one or more genes that have no allele counterparts. Usually applied to genes on the male's X chromosome (in humans). |
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| 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. |
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| hemophilia |
| A sex-linked inherited bleeding disorder that generally only affects males. The disorder is characterized by a tendency to bleed spontaneously or at the slightest injury because of the lack of certain clotting factors in the blood.
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| hepatitis B |
| A potentially serious viral disease that affects the liver; can be transmitted through sexual contact or through contact with infected blood. |
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| herbaceous |
| Term applied to a nonwoody stem/plant with minimal secondary growth. |
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| 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. |
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| heterotrophs |
| Organisms that obtain their nutrition by breaking down organic molecules in foods; include animals and fungi. |
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| heterozygous |
| Possessing two different forms of a particular gene, one inherited from each parent.
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| highly conserved sequence |
| A DNA sequence that is very similar in several different kinds of organisms. Scientists regard these cross species similarities as evidence that a specific gene performs some basic function essential to many forms of life and that evolution has therefore conserved its structure by permitting few mutations to accumulate in it.
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| Hirschsprung's disease |
| A congenital disorder in which part of the colon lacks nerve cells and therefore is unable to relax. The result is chronic constipation and distention of the abdomen. The disorder affects about one in 5000 U.S. newborns but can be treated with surgery.
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| histamine |
| A chemical released during the inþammatory response that increases capillary blood þow in the affected area, causing heat and redness. |
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| histone proteins |
| Proteins associated with DNA in eukaryote chromosomes. |
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| homeobox genes |
| Pattern genes that establish the body plan and position of organs in response to gradients of regulatory molecules. |
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| homeostasis |
| The ability to maintain a relatively constant internal environment. |
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| 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. |
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| 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. |
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| homozygous |
| Possessing two identical forms of a particular gene, one inherited from each parent.
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| hormones |
| Chemical substances that are produced in the endocrine glands and travel in the blood to target organs where they elicit a response. |
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| human artificial chromosome (HAC) |
| A vector used to transfer or express large fragments of human DNA. HACs behave and are constructed like human chromosomes.
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| 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. |
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| Human Genome Project |
| An international research project to map each human gene and to completely sequence human DNA.
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| 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. |
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| human immunodeficiency virus (HIV)/acquired immuno |
| AIDS was first reported in 1981 in the United States and has since become a major epidemic, killing nearly 12 million people and infecting more than 30 million others worldwide. The disease is caused by HIV, a virus that destroys the body's ability to fight infections and certain cancers.
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| 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. |
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| Huntington's disease |
| A degenerative brain disorder that usually appears in mid-life. Its symptoms, which include involuntary movement of the face and limbs, mood swings, and forgetfulness, get worse as the disease progresses. It is generally fatal within 20 years.
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| hybridization |
| Base pairing of two single strands of DNA or RNA.
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| hydrogen bond |
| A weak bond between two atoms (one of which is hydrogen) with partial but opposite electrical charges. |
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| hydrophilic |
| Water-loving. Term applied to polar molecules that can form a hydrogen bond with water. |
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| hydrophobic |
| Water-fearing.Term applied to nonpolar molecules that cannot bond with water. |
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| hydrophytic leaves |
| The leaves of plants that grow in water or under conditions of abundant moisture. |
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| hydrosphere |
| The part of the physical environment that consists of all the liquid and solid water at or near the Earth's surface. |
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| 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. |
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| hypertension |
| High blood pressure; blood pressure consistently above 140/90. |
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| hypertonic |
| A solution having a high concentration of solute. |
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| hyphae |
| The multinucleate or multicellular Þlaments that make up the mycelium (body) of a fungus (sing.: hypha). |
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| 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. |
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| hypothesis |
| An idea that can be experimentally tested; an idea with the lowest level of confidence. |
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| hypotonic |
| A solution having a low concentration of solute. |
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