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Def: Two sequences are homologous if they share a common
evolutionary history, or in other words,
if there existed an ancestral molecule in the past that was ancestral to both of the
sequences.
There is an important difference between similarity and homology. Similarity is observed quantity, that might be for example expressed in percent of identity. For example, sequences might have 30% identity. On the other hand, there are no degrees of homology, as there are of similarity. The sequences are either homologous or not. Types of homology: Orthology: bifurcation in molecular tree reflects speciation. Paralogy: bifurcation in molecular tree reflects gene duplication. Xenology: gene was obtained by organism through horizontal transfer. Synology: genes ended up in one organism through fusion of lineages. |