Glossary
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Glossary Terms for letter: H
habitat The specific area of the natural environment in which an organism lives.
habituate Refers to the process of animals becoming accustomed to human observers.
half-life The time it takes for half of the radioisotopes in a substance to decay; used in various radiometric dating methods.
hand axe The most dominant tool in the Acheulian complex, characterized by a sharp edge for both cutting and scraping.
haplogroups A large set of haplotypes, such as the Y-chromosome or mitochondrial DNA, that may be used to define a population.
haploid A cell that has a single set of unpaired chromosomes, half of the genetic material.
haplotypes A group of alleles that tend to be inherited as a unit due to their closely spaced loci on a single chromosome.
Hardy-Weinberg law of equilibrium A mathematical model in population genetics that reflects the relationship between frequencies of alleles and of genotypes; it can be used to determine whether a population is undergoing evolutionary changes.
heme iron Iron—found in red meat, fish, and poultry—that the body absorbs efficiently.
hemoglobinopathies A group of related genetic blood diseases characterized by abnormal hemoglobin.
hemolytic anemias A condition of insufficient iron in the blood due to the destruction of red blood cells resulting from genetic blood diseases, toxins, or infectious pathogens.
heritability The proportion of phenotypic variation that is due to inheritance rather than to environmental influence.
heteroplasmic Refers to a mixture of more than one type of organellar DNA, such as mitochondrial DNA, within a cell or a single organism's body, usually due to the mutation of the DNA in some organelles but not in others.
heterozygous Refers to the condition in which a pair of alleles at a single locus on homologous chromosomes are different.
homeostasis The maintenance of the internal environment of an organism within an acceptable range.
homeothermic Refers to an organism's ability to maintain a constant body temperature despite great variations in environmental temperature.
homeotic (Hox) genes Also known as homeobox genes, they are responsible for differentiating the specific segments of the body, such as the head, tail, and limbs, during embryological development.
hominids A group of extinct and living bipedal primates in the family Hominidae.
hominin Humans in a more recent evolutionary taxonomy; based on genetics.
Homo erectus An early Homo species and the likely descendant of H. habilis; the first hominid species to move out of Africa into Asia and Europe.
Homo habilis The earliest Homo species, a possible descendant of A. garhi and an ancestor to H. erectus; showed the first substantial increase in brain size and was the first species definitively associated with the production and use of stone tools.
homologous Refers to each set of paired chromosomes in the genome.
homoplasmic Refers to nuclear DNA, which is identical in the nucleus of each cell type (except red blood cells).
homozygous Refers to the condition in which a pair of alleles at a single locus on homologous chromosomes are the same.
Huntington's chorea A rare genetic disease in which the central nervous system degenerates and the individual loses control over voluntary movements, with the symptoms often appearing between ages 30 and 50.
hygiene hypothesis The proposition that increasing allergies among children are the result of decreased exposure to microbes, such as those found in dirt.
hypercholesterolemia The presence of high levels of cholesterol in an organism's blood; this condition may result from the dietary consumption of foods that promote high cholesterol or through the inheritance of a genetic disorder.
hypothermia A condition in which an organism's body temperature falls below the normal range, which may lead to the loss of proper body functions and, eventually, death.
hypotheses Testable statements that potentially explain specific phenomena observed in the natural world.
hypoxia A condition in which an organism is not able to breathe in adequate amounts of oxygen, leading to low levels of oxygen in the blood, shortness of breath, and, in extreme situations, death.
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