Question: per tile sequence quality decreases after QC with sickle
0
gravatar for mary
3.7 years ago by
mary0
mary0 wrote:

I'm trying to QC my illumina miseq paired end reads with sickle pe. but, as visualized in Fastqc per tile sequence quality, it seems that I can not get rid of low quality sequences from special tiles (2118) but somehow it even get worse by increasing quality threshold.

this is the screen-shot from my reads per tile sequence quality after trimming with default quality threshold of sickle pe (q 20):

https://postimg.org/image/s8igtzzc5/

and this is what I get after increasing that to q=30:

https://postimg.org/image/q30kiflbr/

any Idea about what happens around 2118?

ADD COMMENTlink modified 3.7 years ago • written 3.7 years ago by mary0

Instead of dropbox links consider posting the images on imgur.com (or other free image hosting sites) and including the links in your post (use the ctrl+G option when editing the post).

ADD REPLYlink written 3.7 years ago by genomax91k
1
gravatar for Brian Bushnell
3.7 years ago by
Walnut Creek, USA
Brian Bushnell17k wrote:

I suspect it turns red if 100% of the reads are gone from a tile, in which case the calculated average quality would be 0 to avoid a division by zero problem.

For this kind of issue, I suggest using FilterByTile rather than blanket-applying extremely high quality-trimming thresholds like Q30 to your entire dataset, which incurs bias and can greatly damage the utility of the data for many purposes.

ADD COMMENTlink modified 3.7 years ago • written 3.7 years ago by Brian Bushnell17k

seems reasonable! thanks for your help!

ADD REPLYlink written 3.7 years ago by mary0
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