IGV reads (grey vs white)

[ocs]

Hello folks,

concerning the Integrative Genomics Viewer:
What is the difference between the white and the grey reads? Some reads in the alignment are all filled with grey and some are outlined with grey but filled with white. I couldn't find any answer in the manual (http://www.broadinstitute.org/softwa.../AlignmentData).

Thanks in advance,
best regards,
Oliver

[Seq84]

Hi,
Reads filled with white have mapping quality score = 0
best regards,
Matteo

[ocs]

Thanks!
Does this mean that they are aligned just by desperation?

[nilshomer]

It usually means there are two or more equally likely locations, and so this location was randomly chosen from that set.

[ocs]

Hello nilshomer,

thanks for your answer. Its a bit confusing, I can think of two equally likly locations that both have a great mapping quality but then its set to zero because its ambiguous?

[Jim Robinson]

First apologies this was not documented, we'll correct that. I agree its confusing, I think ambiguous mappings should be indicated by a separate score or field, but that's the way it is.

Jim

[Orr Shomroni]

Hello everyone,

To continue on this grey vs. white issue, I have an IGV plot containing 2 reads: 1 long at a certain position, and another slightly shorter which is one position afterwards (i.e. in terms of positions, the small read is contained within the range of the big read). In IGV, the big read is marked white (i.e. can be aligned elsewhere), the small is grey, and the big read has no SNPs/Indels in its edges. The question is, how is it possible that a certain read can be aligned to multiple positions, whereas its subsequence is unique?
On the same note, which tool determines whether a read can be mapped to multiple locations (i.e. white bar)? Can IGV calculate that itself?

[Jim Robinson]

Hi,

IGV is not making any judgement calls, is is using the mapping quality. It is a convention with at least some aligners (BWA), if not all aligners, that mapping quality == 0 means the read is not uniquely mapped. IGV does not do any calculation. So if you want to take all interpretation out of the picture a "white" read simply means that the mapping quality has been set to zero by the aligner.

Jim