--- title: "Sequence alignment and reading a search result" description: "Alignments receive scores for matches, mismatches and gaps under stated rules. BLAST can report an E-value: the expected number of chance alignments with that score or higher in the search space. Stat" canonical: https://lightmysky.com/learn/science/sequence-alignment-and-reading-a-search-result-mt_jNbEMFqgZ5 source: https://lightmysky.com/learn/science/sequence-alignment-and-reading-a-search-result-mt_jNbEMFqgZ5.md retrieved: 2026-11-12 --- > **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`. # Sequence alignment and reading a search result Alignments receive scores for matches, mismatches and gaps under stated rules. BLAST can report an E-value: the expected number of chance alignments with that score or higher in the search space. Statistical support alone does not establish biological function. Subject: Science · Area: Genetics & Evolution · Ages 20 to 22 Page: https://lightmysky.com/learn/science/sequence-alignment-and-reading-a-search-result-mt_jNbEMFqgZ5 ## Ready when they can - Scores a short alignment by hand under a stated scheme, gaps included. - Reads a search result and separates a strong hit from a chance one using the reported statistic. - Explains why a short query can have weak statistical support even for a perfect match. ## Lesson: Scoring alignments and judging hits Assembly uses shared sequence; alignment makes a comparison explicit. Align DNA, RNA or protein sequences in columns and score matches, mismatches and gaps. Among candidates using the same scoring scheme and constraints, prefer the highest total. Reads can be aligned before a genome is assembled. For our DNA examples, use +2 per matching column, -1 per mismatch and -2 per gap character. Count every displayed column, including end gaps. No column has two gaps. Other schemes can charge separately for opening and extending a gap, so read the stated rules. **Example.** Four matches, one mismatch and one gap character score 4 times 2, minus 1, minus 2: a total of 5. Count each category, multiply by its score and add the signed contributions. Prefer the higher total when candidates use the same rules and constraints. *(drawing: For ACGT aligned with A-GT, the column scores add to 2 - 2 + 2 + 2 = 4.)* BLAST reports an E-value: the expected number of chance alignments with that score or higher in the search space. It is an expected count, not the probability that a hit is false. Smaller values give stronger evidence against chance under the model. Check how much of the query aligns and whether the matched region supports a proposed function. A short perfect match can still have a large E-value because few letters can match by chance. E = 5 means five alignments at least this strong are expected by chance in that search space. Perfect identity alone is then weak evidence. Length, composition, database size and scoring affect the statistic. **Recap.** Use explicit scoring rules. An E-value reports expected chance matches; aligned length and biological evidence still matter. *Lesson, questions and flashcards written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Sequencing reads and how a genome is assembled](https://lightmysky.com/learn/science/sequencing-reads-and-how-a-genome-is-assembled-mt_4mUJIUAZGd) ## Opens up - [Annotation, orthologues and comparative genomics](https://lightmysky.com/learn/science/annotation-orthologues-and-comparative-genomics-mt_eBRge2bLeC) - [Alignment Scoring: Substitution Matrices, Gap Penalties and Significance](https://lightmysky.com/learn/science/alignment-scoring-substitution-matrices-gap-penalties-and-significance-mt_k--A8iF5SC) - [Distance and Likelihood Trees, and What Branch Support Means](https://lightmysky.com/learn/science/distance-and-likelihood-trees-and-what-branch-support-means-mt_mEO9PUEdh6)