nucleotide sequences that are products of from a common ancestor.
If one gene, as found in one species, remains in two or more lineages following , at the same locus in , then the resulting genes are described as orthologous. In each case the vertical inheritance would be from parent to offspring in an unbroken string from common ancestor species to the various descendant species carrying the orthologous genes.
Another way to look at orthologous genes is that these are products of relatively uncomplicated evolution, i.e., the assumed default circumstance where genes in remain intact, if divergent, such that one can characterize both genes along with their locations in genomes and be reasonably certain that these genes are effectively the "same" gene as found in different species. Orthologous genes, that is, are in terms of both structure and location plus share an equivalent if now divergent .
Contrast with paralogous genes.