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 s