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Question: More than one possible mapping place for a read
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gravatar for biotech
3.7 years ago by
biotech • 540
United States
biotech • 540 wrote:

When there is more than one possible mapping place for a read, being both places of equal score,

Where 'bwa mem' mapper finally maps it?

What about the mapping quality scores for the read?
bwa • 849 views
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modified 3.7 years ago by WouterDeCoster ♦ 42k • written 3.7 years ago by biotech • 540
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gravatar for WouterDeCoster
3.7 years ago by
WouterDeCoster ♦ 42k
Belgium
WouterDeCoster ♦ 42k wrote:

It depends on your settings, but I think the default is multimapping the reads. So the same read will be present in multiple locations. Mapping quality will be 0 (meaning the read maps equally well in multiple places).
ADD COMMENT • link written 3.7 years ago by WouterDeCoster ♦ 42k

Are you certain about multi-mapping? bwa mem manual entry does not say anything specific but the older sampe/samse explicitly say that

Repetitive read pairs will be placed randomly.

Repetitive hits will be randomly chosen.

ADD REPLY • link written 3.7 years ago by genomax ♦ 75k
1

You are giving me doubts about my answer. ;-) Might very well be possible that random mapping happens.

Well, the easiest approach to settle this would be to test it. Have a single multimapping read, map it, count the reads in the samfile.

ADD REPLY • link written 3.7 years ago by WouterDeCoster ♦ 42k

I really don't care about random mapping as long as having mapping quality of zero for them. In fact, multi-mapping reads should be discarded for some analysis such as RNA-seq.

ADD REPLY • link written 3.7 years ago by biotech • 540

Great proof test @decosterwouter, I will try to see what's going on!

ADD REPLY • link written 3.6 years ago by biotech • 540

I agree about that mapping quality is 0, but not sure if repetitive read pairs will be placed randomly.

ADD REPLY • link written 3.7 years ago by biotech • 540
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