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Haplotype analylsis. Molecular genetic testing to identify a set of closely linked segments of DNA.

Health Maintenance Organization (HMO). A type of health insurance plan that usually limits coverage to care from doctors who work for or contract with the HMO. It generally won’t cover out-of-network care except in an emergency.

Heterotropia. Inability of one eye to attain binocular vision with the other because of imbalance of the muscles of the eyeball--called also strabismus, squint.

Hydramnios. Excessive amniotic fluid build-up during pregnancy.

Hyperkalemia. A greater than normal concentration of potassium ions in the circulating blood.

Hyperostosis. Excessive growth of bony tissue.

Hypersomnia. Excessive daytime sleepiness.

Hypertrophy. General increase in bulk or a part of an organ.

Hypothermia. A body temperature significantly below 98.6.

Hypotonia. Low muscle tone causing floppiness, as in a child with the congenital form of myotonic dystrophy.