Thank you so much for clearing that up. I was unaware that was the
convention when only one paired end is mapped.
On Tue, May 27, 2014 at 6:56 AM, John Marshall <j...@sanger.ac.uk> wrote:
> On 27 May 2014, at 10:58, Wolfgang Maier <
> wolfgang.ma...@biologie.uni-freiburg.de> wrote:
> > On 22.05.2014 20:59, Kate Im wrote:
> >> the number of unmapped read (estimated by
> >> subtracting the reported number of mapped reads from the reported number
> >> of total reads) is always higher than the number of sequences with an
> "*"
> >> in the third column of the SAM file. Shouldn't these be the same?
> >
> > Ideally, yes, but the SAM/BAM format specifications
> > (http://samtools.github.io/hts-specs/SAMv1.pdf) say that:
> >
> > "Bit 0x4 [in the FLAG field] is the only reliable place to tell whether
> > the segment is unmapped.
>
> In particular (see ยง2, 4.1 of that document), there is the common
> convention for pairs in which just one end is mapped, of giving both reads
> the RNAME and POS (3rd and 4th) columns of the mapped end. This has the
> useful side-effect of bringing the unmapped end alongside its mate when the
> file is coordinate-sorted.
>
> John
>
> --
> The Wellcome Trust Sanger Institute is operated by Genome Research
> Limited, a charity registered in England with number 1021457 and a
> company registered in England with number 2742969, whose registered
> office is 215 Euston Road, London, NW1 2BE.
>
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