Orthology and Homology

Most genes are found in many species, doing similar functions in each one. By tracking genes across species, we can compare their functions in different species, and we can also compare the genomes of many species by what genes they do and do not have.

Definitions

Finding the equivalent gene in different species can be tricky, and the terminology doesn't always help. Any pair of genes that are related to each other are called Homologs. Orthologs are genes that are related by speciation (e.g. the same gene in chimp and human). The difference between the two is caused by gene duplication: for instance, humans have four EGFR genes, due to a double duplication in early vertebrates, while insects have only one. Those four duplicates tend to have somewhat different functions (otherwise, they would have been lost during evolution). EGFR1 and EGFR2 in human are homologs but not orthologs (they didn't split due to speciation), and so they are called paralogs. Similarly, EGFR1 in human and EGFR2 in mouse are paralogs (they are in different species, but their original split came from gene duplication, not speciation).

What about the relationship between insect EGFR and human EGFR1? These have had both a duplication and a speciation, and should have more function in common than human EGFR1 has to EGFR2. Some people call these in-paralogs, but others consider them as orthologs, or sometimes co-orthologs.

Orthology Resources

An annotated list of orthology methods and databases on the web.