Literally meaning: “straight homolog"
Origin: Anc Greek
ορθο-/ortho- (=straight, correct, direct)
λόγος/logos(=word, speech, discourse, reason)
The terms "ortholog" and "paralog" were coined in 1970 by American biologist Walter Fitch (1929 – 2011) in order to differentiate between different kinds of homology and different subsets of homologs. The homologous characters (genes and their products) that are produced by speciation are called orthologs meaning “straight homology”. Homologous genes that are produced as gene duplication, which can occur within a lineage, in the absence of any speciation are called paralogs meaning “parallel homologs”.
Orthologs are genes in different species originating from a single ancestral gene in the last common ancestor of the compared genomes. Orthologs retain the same function whereas paralogs evolve new functions.