U

Ultracentrifuge: A very high-speed centrifuge used to separate macromolecules.
Ultrameric tree: A phylogenetic tree in which the branch lengths are constrained to all be equidistant from the root. It is also known as a dendrogram.
Uncultured microbes: Microbes that have never successfully been grown in isolation in the laboratory.
Underdominance: Describes heterozygotes that have lower trait values (usually lower fitness) than either homozygote.
Undernourishment: A diet that is chronically deficient in calories.
Unequal crossover: The outcome when two tandemly repeats sequences do not pair correctly. A cross-over between them produces one genome with fewer repeats and one with more.
Uniformitarianism: The assumption that the same natural processes acted in the past as are observed to operate now.
Unique event polymorphism: An allele for which all copies derive from a single mutational event.
Universal homology: A homologous trait found in all cellular organisms.
Unrooted tree: A phylogenetic tree in which the root is not shown (frequently because it is not known).
Unsaturated fatty acid: Fatty acid possessing one or more double bonds between the carbons in the hydrocarbon tail. Such bonding reduces the number of hydrogen atoms attached tot he carbon skeleton.
Urea: A soluble nitrogenous waste excreted by mammals, most adult amphibians, sharks, and some marine bony fishes and turtles; produced in the liver by a metabolic cycle that combines ammonia with carbon dioxide.
Ureter: A duct leading from the kidney to the urinary bladder.
Urethra: A tube that releases urine from the body near the vagina in females and through the penis in males.; also serves in males as the exit tube for the reproductive system.
Uric acid: An insoluble precipitate of nitrogenous waste excreted by land snails, insects, and many reptiles, including birds.
Urinary bladder: The pouch where urine is stored prior to elimination.
Uterine cycle: The changes that occur in the uterus during the reproductive cycle of the human female; also called menstrual cycle.
Uterus: A female organ where eggs are fertilized and/or the development of the young occurs.
Utricle: A chamber behind the oval window that opens into the three semicircular canals.