Audio Glossary

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  1. habitat: A characteristic place or type of environment in which an organism lives.
  2. haploid: haploid Of or referring to a cell or organism that has only one complete set of homologous chromosomes (n). Compare diploid.
  3. Hardy–Weinberg equation: An equation (p2 + 2pq + q2 = 1) that predicts the genotype frequencies in a population that is not evolving.
  4. heart: The muscular pump that pushes blood through the animal circulatory system.
  5. heart rate: The number of heartbeats per minute.
  6. heartbeat: The series of events corresponding to one cardiac cycle.
  7. heartwood: Older, centrally located secondary xylem tissue that provides support to a tree but has become clogged over time and hence no longer functions in the transport of materials. Compare sapwood.
  8. helper T cell: A type of lymphocyte that stimulates killer T cells and B cells.
  9. hemoglobin: An oxygen-binding pigment that is used to carry oxygen from the gas exchange surfaces to the tissues.
  10. herbivore: A consumer that relies on living plant tissues for nutrients. Compare carnivore.
  11. hermaphrodite: An individual that can produce both eggs and sperm.
  12. heterozygote: An individual that carries one copy of each of two different alleles (for example, an Aa individual). Compare homozygote.
  13. high-density lipoprotein (HDL): A molecule that carries excess cholesterol through the circulatory system to the liver. Compare low-density lipoprotein.
  14. hindbrain: The portion of the brain that includes the medulla oblongata, the pons, and the cerebellum and coordinates breathing rhythms and balance.
  15. histamine: A protein released by mast cells that causes blood vessels to dilate and capillaries to become more porous.
  16. histone spool: A group of proteins around which the DNA of a eukaryotic chromosome is wound.
  17. homeostasis: The process of maintaining appropriate and constant conditions inside cells.
  18. homeotic gene: A master-switch gene that plays a key role in the control of gene expression during development. Each homeotic gene controls the expression of a series of other genes whose protein products direct the development of an organism.
  19. hominoid: Any of a group of primates that encompasses humans and our now extinct humanlike ancestors.
  20. homologous: Of or referring to a characteristic shared by two groups of organisms because of their descent from a common ancestor. Compare analogous.
  21. homologous chromosomes: The two members of a specific chromosome pair found in diploid cells, one of which comes from the individual's mother and the other from its father.
  22. homologue: One of a pair of homologous chromosomes.
  23. homozygote: An individual that carries two copies of the same allele (for example, an AA or an aa individual). Compare heterozygote.
  24. horizontal gene transfer: The movement of genes from one organism or group of organisms to another, not vertically within a lineage via reproduction, but horizontally to another lineage altogether by some other means.
  25. hormone: A signaling molecule released into the circulatory system of an animal or the vascular system of a plant in very small amounts that affects the functioning of target tissues.
  26. host: An organism in which a parasite or pathogen lives.
  27. housekeeping gene: A gene that has an essential role in the maintenance of cellular activities and is expressed by most cells in the body.
  28. Human Genome Project (HGP): A publicly funded effort on the part of an international consortium created by the U.S. National Institutes for Health and the U.S. Department of Energy to determine the sequence of the human genome.
  29. humoral immunity: A form of immunity that relies on the marking of pathogens by the antibodies produced by B cells. Compare cell-mediated immunity.
  30. hybrid: An offspring that results when two different species mate.
  31. hybridize: To cause hybrid offspring to be produced.
  32. hydrogen bond: A chemical linkage between a hydrogen atom, which has a slight positive charge, and another atom with a slight negative charge. Compare covalent bond and ionic bond.
  33. hydrophilic: Of or referring to molecules or parts of molecules that interact freely with water. Hydrophilic molecules dissolve easily in water, but not in fats or oils. Compare hydrophobic.
  34. hydrophobic: Of or referring to molecules or parts of molecules that do not interact freely with water. Hydrophobic molecules dissolve easily in fats and oils, but not in water. Compare hydrophilic.
  35. hydrostat: A support structure that depends on the interaction between a fluid under pressure and an elastic membrane for stiffness.
  36. hypertension: Abnormally high blood pressure.
  37. hypertonic solution: A solution that has a higher solute concentration than the cytosol of a cell, causing more water to flow out of the cell than into it. Compare hypotonic solution and isotonic solution.
  38. hypha (pl. hyphae): In fungi, a threadlike absorptive structure that grows through a food source. Mats of hyphae form mycelia, the main bodies of fungi.
  39. hypothalamus (pl. hypothalami): A structure at the base of the vertebrate brain that controls the release of hormones by the pituitary gland. Along with the pituitary gland, the hypothalamus helps regulate interactions between the nervous and endocrine systems.
  40. hypothesis (pl. hypotheses): A possible explanation of how a natural phenomenon works. A hypothesis must have logical consequences that can be proved true or false.
  41. hypotonic solution: A solution that has a lower solute concentration than the cytosol of a cell, causing more water to flow into the cell than out of it. Compare hypertonic solution and isotonic solution.