The relationship between the elements in the same group of the periodic table, or between organic compounds in a homologous series
A correspondence of structures in two life forms with a common evolutionary origin, such as flippers and hands
similarity in the sequence of a protein or nucleic acid or in structure of an organ that reflects a common evolutionary origin
Being related by the evolutionary process of divergence from a common ancestor Homology implies similarity, but low similarity does not necessarily prove homology See also: Twilight zone, Midnight zone
Homologs have common origins but may or may not have common activity Genes that share an arbitrary threshold level of similarity determined by alignment of matching bases are termed homologous They are inherited from a common ancestor which possessed the structure This may be difficult to determine when the structure has been modified through descent Note: homology is a qualitative term (something is either homologous or not), while similarity is the corresponding quantitative term Therefore, it is more appropriate to refer to xx % similarity (i e never xx % homology)
1 The relationship of any two characters that have descended from a common ancestor This term can apply to a morphological structure, a chromosome or an individual gene or DNA segment 2 In MGI, assertions of Mammalian Homology imply presumed orthology See the Figure at NCBI See also: Homolog, Orthology, Paralogy
The correspondence or resemblance of substances belonging to the same type or series; a similarity of composition varying by a small, regular difference, and usually attended by a regular variation in physical properties; as, there is an homology between methane, CH4, ethane, C2H6, propane, C3H8, etc
Clad Any characteristic shared between two that whose similarity is due to common descent Opposed to homoplasy In cladistics, both homology and homoplasy are determined a posteriori with reference to a particular phylogeny that maximizes homology and minimizes homoplasy
The quality of being homologous; correspondence; relation; as, the homologyof similar polygons
Similarity of the structure, physiology, or development of different species of organisms based on their descent from a common evolutionary ancestor. Analogy, by contrast, is a functional similarity of structure that is based not on common evolutionary origins but on mere similarity of use. The forelimbs of such widely differing mammals as humans, bats, and deer are homologous; the form of construction and the number of bones in each are practically identical and represent adaptive modifications of the forelimb structure of their shared ancestor. The wings of birds and insects, on the other hand, are merely analogous; they are used for flight in both types of organisms but do not share a common ancestral origin
the quality of being similar or corresponding in position or value or structure or function
Two structures are considered homologous when they are inherited from a common ancestor which possessed the structure This may be difficult to determine when the structure has been modified through descent