# DBT JRF 2022 — Question 126

Exam: DBT JRF 2022 · Section: B · Topic: Bioinformatics › BLAST Search & Statistical Significance · Difficulty: Medium

## Question

Your friend runs a BLASTp on his unknown sequence and finds a match with E-value of 3 with a target. What does this mean?

## Options

- **A.** There are $e^3$ sequences in the database that match the unknown sequence.
- **B.** There are 3 mismatches between the unknown and target sequences.
- **C.** The random chance of unknown sequence matching the target is 1 in 3.
- **D.** There are 3 hits of similar score that can be found by random chance.

## Answer

Option D

## Explanation

BLASTp E-value of 3 — interpretation: The E-value (Expect value) represents the number of alignments of equal or better score expected by CHANCE alone in a database of that size: • E-value = 3 means: You would expect to find 3 hits of this score or better just by random chance. • This is a HIGH E-value → the match is NOT statistically significant → likely a coincidental match. Common misconceptions: • E-value ≠ number of mismatches: mismatches affect the alignment SCORE (not directly the E-value). • E-value is NOT a probability in the 0–1 range (it CAN be >1). • E-value ≠ '1 in 3 chance': the probability interpretation would be $P \approx 1 - e^{-E}$ ; for small E, $P \approx E$ , but for E=3, $P = 1 - e^{-3} \approx 0.95$ . Conclusion: E-value = 3 means 3 random hits expected by chance alone → this match is NOT biologically meaningful. Answer: E-value of 3 means 3 similar hits are expected by random chance; the match is NOT significant.

Source: https://mypyqbuddy.com/questions/your-friend-runs-a-blastp-on-his-unknown-6a104cd5920734408bc0c78d