Paralogous genes

Paralogous genes (or paralogs) are genes that are homologous (share a common ancestry) but were separated by a gene duplication event. If a gene, G1, is duplicated to give a copy, G1A, then G1 and G1A are paralogs of one another. Paralogs often have the same or similar functions, but not always: due to the lack of original selective pressure on the gene copy, it has relative freedom to mutate and may adopt a new function. It should be noted, however, that the term 'paralog' does not concern function but exclusively relationship between two or more genes.

Paralogs may be found in the same or different genomes depending on whether the duplication occurred before or after a speciation event. The paragraph below illustrates examples of paralogous (and orthologous) genes in both the same and in different genomes.

The same genome:
If a species, S1, has a gene, G1, and S1 gives rise to two more species, S2 and S3, each with genes, G2 and G3, then G2 and G3 are orthologs of one another. However, if there is a gene duplication event in S2 to give rise to a gene copy, G2A, then G2 and G2A are paralogs in the same genome and both are orthologs of gene G3. Here gene duplication has occurred after speciation, hence the paralogs existing in the same genome.

Different genomes:
If the species, S1, the common ancestor of S2 and S3, has two gene duplication events to produce G1A and G1B from G1, then a speciation event occurs which separates these genes - G1A in S2 and G1B in S3 - then G1A and G1B are still considered paralogs of one another, despite existing in separate genomes. They are not orthologs, because the original root of their homology was a gene duplication event and not a speciation event. When gene duplication occurs before speciation, paralogs may exist in different genomes.