Bioinformatics Glossary


Q20 (Quality score 20)
A quality score of > or = 20 indicates that there is less than a 1 in 100 chance that the base call is incorrect. These are consequently high-quality bases. Specifically, the quality value "q" assigned to a basecall is defined as:
q = -10 x log10(p)
where p is the estimated error probability for that basecall. Note that high quality values correspond to low error probabilities, and conversely.

Quality trimming
This is an algorithm which uses a sliding window of 50 bases and trims from the 5' end of the read followed by the 3' end. With each window, the number of low quality (10 or less) bases is determined. If more than 5 bases are below the threshold quality, the window is incremented by one base and the process is repeated. When the low quality test fails, the position where it stopped is recorded. The parameters for window length low quality threshold and number of low quality bases tolerated are fixed. The positions of the 5' and 3' boundaries of the quality region are noted in the plot of quality values presented in the" Chromatogram Details" report.

Query (待查序列/搜索序列)
The input sequence (or other type of search term) with which all of the entries in a database are to be compared.

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