--- title: "Annotation, Orthologues and Comparative Genomics" description: "An assembled genome is only useful once genes are located and their likely functions assigned, usually by similarity to genes already studied elsewhere. Comparing species shows which sequences have be" canonical: https://lightmysky.com/learn/science/annotation-orthologues-and-comparative-genomics-mt_eBRge2bLeC source: https://lightmysky.com/learn/science/annotation-orthologues-and-comparative-genomics-mt_eBRge2bLeC.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Annotation, Orthologues and Comparative Genomics An assembled genome is only useful once genes are located and their likely functions assigned, usually by similarity to genes already studied elsewhere. Comparing species shows which sequences have been conserved, which is a strong hint that they matter. Subject: Science · Area: Genetics & Evolution · Ages 21 to 22 Page: https://lightmysky.com/learn/science/annotation-orthologues-and-comparative-genomics-mt_eBRge2bLeC ## Ready when they can - Distinguishes genes related by speciation from genes related by duplication, and says why the difference matters for function. - Uses conservation across distant species as evidence of function, and states the limits of that argument. - Explains why an annotation transferred by similarity is a hypothesis rather than a result. ## Lesson: Same genes, different species, shared jobs An assembled genome is only useful once genes are located and their likely functions assigned. New genes are usually named by matching them to genes already studied in other species. That transfer is powerful but provisional. A match says the two genes look alike, not that they do the same thing. Every transferred annotation is a prediction waiting for a test. Orthologues are genes in different species that descend from one gene in their last common ancestor. They split because the species split, and they usually keep the ancestral job. Paralogues instead come from a duplication inside one genome, leaving two or more copies in the same lineage. The spare copies are free to mutate and take on new jobs. Trees built from shared DNA tell the two histories apart. Line genomes up side by side and some stretches match closely across distant relatives. Those conserved stretches flag DNA that evolution refused to change. If a sequence is unchanged, mutations there were probably harmful, which suggests the sequence does something important. But the argument has limits: some sequences persist for structural reasons, and similarity alone never proves function. Only experiments settle it. **Tip.** A conserved non-coding sequence is worth investigating, since it may regulate a nearby gene. Compare the same region across species and ask which parts stayed put over millions of years. Genes that diverged with the species should track the species tree. Reading both trees together shows which similarities mean shared function and which do not. **Recap.** Annotation by similarity is a hypothesis, orthologues usually keep the old job while duplicates wander, and conservation flags sequences worth testing. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Sequence Alignment and Reading a Search Result](https://lightmysky.com/learn/science/sequence-alignment-and-reading-a-search-result-mt_jNbEMFqgZ5) ## Opens up - [Association Studies and What They Can and Cannot Show](https://lightmysky.com/learn/science/association-studies-and-what-they-can-and-cannot-show-mt_IFFUFX3xrv) - [From a Gene List to Biology: Enrichment Tests and Their Backgrounds](https://lightmysky.com/learn/science/from-a-gene-list-to-biology-enrichment-tests-and-their-backgrounds-mt_M9eHEXq6iA)