Probable homologue for a given parental sequence, but may not have arisen by direct ancestry for example could have arisen indirectly for example through gene duplication

To understand an orthologue it is first better to consider a homologue. A homologue means each residue of DNA or amino acid has a shares direct ancestory with the same residue in a separate genome. Thus if the common ancestor mutated, each ancestor would carry the same mutation at exactly the same position. In other words we are confident this represents inheritance. An orthologue is a likely ancestor, but may not share direct ancestry for example if a gene has duplicated one genome may have inherited one copy a different genome inherited a different copy and therefore homology can not be assumed. In this case the sequences are considered orthologues.