E-value and query coverage
Wed, Sep 16, 2026 · Week 4 · Genetics of Disease (Medical Interventions)
Today's goal: Interpret E-value and query coverage to judge how trustworthy a BLAST match really is.
What a finished product looks like
This is a model of the work you should turn in today. Use it to check your own: match the structure and the level of detail, do not copy it. Your data and wording should be your own.
Best match: Staphylococcus aureus.
I chose it because it has the lowest E-value (2e-145, which is far closer to zero than any other row), the highest query coverage (99%, so almost my whole sequence aligned), and the highest percent identity (99.4%). Staphylococcus epidermidis is close, but it is lower on all three numbers, so it is the second choice, not the call. Bacillus subtilis (E-value 0.8) and E. coli (E-value 4.5) have E-values near or above 1, which means a match that good could easily happen by chance, so they are not trustworthy identifications.
| Match | Query coverage | E-value | Identity |
|---|---|---|---|
| Staphylococcus aureus | 99% | 2e-145 | 99.4% |
| Staphylococcus epidermidis | 97% | 3e-120 | 94.1% |
| Bacillus subtilis | 41% | 0.8 | 78.0% |
| Escherichia coli | 22% | 4.5 | 71.2% |
Also due today: Paste the screenshot of your BLAST hit table into your evidence log and write the one-sentence claim naming your pathogen.
WebXam problem for today's skill
One exam-style question that uses exactly what you practiced today. Try it before you reveal the answer, then read why each choice is right or wrong.
| Option | Query coverage | E-value | Identity |
|---|---|---|---|
| Staphylococcus aureus | 99% | 2e-145 | 99.4% |
| Staphylococcus epidermidis | 97% | 3e-120 | 94.1% |
| Bacillus subtilis | 41% | 0.8 | 78.0% |
| Escherichia coli | 22% | 4.5 | 71.2% |
Tap an answer to see the full explanation. Nothing is recorded or graded.