Homologous Genes definition
Homology derives from the Greek homos (equals) and logos (relation) and refers to the relation existing between two structures (of different species) that share a common ancestor. Definition of homologous genes is based on the same, which states that two genes are homologous when their DNA sequence derives from a common origin, and may or may not have the same function. In contrast, definition of analogous genes concerns two genes that have the same function but do not share the same origin. That said, homologous genes are considered to be the product of a divergent evolution, while analogous genes have emerged from a convergent evolution.
Homologous genes can originate through three distinct events:
- speciation event, which gives rise to orthologous genes;
- genetic duplication event, which gives rise to paralogous genes;
- horizontal gene transfer event, which gives rise to xenologous genes.
Orthologous genes
Two homologous genes are designated orthologs when they were separated by a speciation event, where each copy of the gene diverged to two distinct species. Thus, orthologous genes share a common ancestor and have equal functions. Discovery of orthologous genes is very important in the study and determination of function of new sequenced genes. The term “ortholog” was first defined and used in 1970 by Walter Fitch.
Paralogous genes
Two homologous genes are designated paralogs when they were originated by a doubling event within the genome of the same species, occupying different positions in the same genome. Consequently, paralogous genes share a common ancestor, yet they have distinct functions. The study of paralogous genes is an important and useful tool in the study of species evolution.
Xenologous genes
For xenologous genes, they originate when homologous genes are the product of horizontal gene transfer between two species and, depending on the new environment for which gene has been moved, they may have similar or distinct functions.
