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.
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).