Sequences are said to be homologous when they share a common ancestral sequence. If several organisms each contain single copies of a particular gene that all derive from the same common ancestor then sequence comparison should give an accurate evolutionary tree. However, gene duplication may result in multiple copies of the same gene within a single organism. These alternatives are illustrated in Fig. 20.13. Orthologous genes are those found in separate species and which diverged when the organisms containing them diverged. Paralogous genes are multiple copies located within the same organism due to gene duplication.
To generate an accurate evolutionary tree orthologous genes must be compared. For example, we must compare the sequences of a-globin from one animal with the orthologous a-globin from another and not with the paralogous b-globin. Since paral-ogous sets of genes have similar sequences, these may cause confusion unless their homologous Related in sequence to an extent that implies common genetic ancestry orthologous genes Homologous genes that are found in separate species and which diverged when the organisms containing them diverged paralogous genes Homologous genes that are located within the same organism due to gene duplication
Orthologs i 1
Was this article helpful?