Hardy-Weinberg law A basic concept in population genetics that relates the gene frequency to the genotype frequency. The Hardy-Weinberg law can be used, for example, to determine allele frequency and heterozygote frequency when the incidence of a genetic disorder is known.
hard palate The first section of the bony part of the roof of the mouth, located in front of the soft palate.
hard measles See measles.
haplotype 1 A set of closely linked genetic markers present on one chromosome that tend to be inherited as a unit, such as those genes of the major histocompatibility complex (MHC). 2 A phenotype (set of characteristics) determined by closely linked genes inherited as a unit from one parent.
haploid A set of chromosomes that contains only one member of each chromosome pair. The sperm and egg are haploid and, in humans, each has 23 chromosomes.
hantavirus pulmonary syndrome A severe lung condition caused by hantavirus infection. Abbreviated HPS. In 1993, HPS struck the Four Corners area (where the states of Arizona, New Mexico, Nevada, and Utah meet) with devastating, frequently fatal consequences. As the name indicates, HPS is due to a hantavirus. The HPS outbreak in Four Corners followed 2…
hantavirus A group of viruses that cause hemorrhagic fever and pneumonia. The hantaviruses include the hantaan virus that causes Korean (and Manchurian) hemorrhagic fever. Hantaviruses are transmitted to humans by direct or indirect contact with the saliva and excreta of rodents, such as deer mice, field mice, and ground voles.
Hansen’s disease See leprosy.
Hangman’s fracture A broken neck involving a fracture of an upper cervical vertebra similar to the injury suffered in death by hanging. The fracture affects the vertebra called the axis, the second cervical vertebra (C2), with or without subluxation (slippage) of C2 on the third cervical vertebra (C3).
Hand-Schuller-Christian disease A form of Langerhans cell histiocytosis usually affecting children aged 2 to 5. The condition is characterized by the accumulation of histiocytes (specialized cells of the immune system) in various organs, causing damage to the tissues. Involvement of bones is characteristic, and the most frequent sites of bony involvement are the flat bones…
hand-foot-and-mouth disease A common viral illness usually seen in infants and children characterized by a rash on the hands and feet and in the mouth. The internal rash (enanthem) consists of blisters and little ulcers. These may involve not only the lining of the mouth but also the gums, palate, and tongue. The external rash…
hamstring One of the prominent tendons at the back of the knee. The hamstrings are the side walls of the hollow behind the knee (popliteal space). Both hamstrings connect to muscles that flex the knee. The term is sometimes used to refer to the muscles in the back of the thigh. A pulled hamstring is…