Right now, if you select a multi-sample VCF file down (or one with filters I see) down to a smaller set of samples, and the site isn't polymorphic in that subgroup, then the alt allele is lost. For example, when selecting down NA12878 from the OMNI, I previously received the following VCF:
1 82154 rs4477212 A . . PASS AC=0;AF=0.00;AN=2;CR=100.0;DP=0;GentrainScore=0.7826;HW=1.0 GT:GC 0/0:0.7205
1 534247 SNP1-524110 C . . PASS AC=0;AF=0.00;AN=2;CR=99.93414;DP=0;GentrainScore=0.7423;HW=1.0 GT:GC 0/0:0.6491
1 565286 SNP1-555149 C T . PASS AC=2;AF=1.00;AN=2;CR=98.8266;DP=0;GentrainScore=0.7029;HW=1.0 GT:GC 1/1:0.3471
1 569624 SNP1-559487 T C . PASS AC=2;AF=1.00;AN=2;CR=97.8022;DP=0;GentrainScore=0.8070;HW=1.0 GT:GC 1/1:0.3942
Where the first two records lost the ALT allele, because NA12878 is hom-ref at this site. My change results in a VCF that looks like:
1 82154 rs4477212 A G . PASS AC=0;AF=0.00;AN=2;CR=100.0;DP=0;GentrainScore=0.7826;HW=1.0 GT:GC 0/0:0.7205
1 534247 SNP1-524110 C T . PASS AC=0;AF=0.00;AN=2;CR=99.93414;DP=0;GentrainScore=0.7423;HW=1.0 GT:GC 0/0:0.6491
1 565286 SNP1-555149 C T . PASS AC=2;AF=1.00;AN=2;CR=98.8266;DP=0;GentrainScore=0.7029;HW=1.0 GT:GC 1/1:0.3471
1 569624 SNP1-559487 T C . PASS AC=2;AF=1.00;AN=2;CR=97.8022;DP=0;GentrainScore=0.8070;HW=1.0 GT:GC 1/1:0.3942
The genotype remains unchanged, but the ALT allele is now preserved. I think this is the correct behavior, as reducing samples down shouldn't change the character of the site, only the AC in the subpopulation. This is related to the tricky issue of isPolymorphic() vs. isVariant().
isVariant => is there an ALT allele?
isPolymorphic => is some sample non-ref in the samples?
In part this is complicated as the semantics of sites-only VCFs, where ALT = . is used to mean not-polymorphic. Unfortunately, I just don't think there's a consistent convention right now, but it might be worth at some point to adopt a single approach to handling this. Wiki docs updated.
Does anyone have critical infrastructure that depends on the previous convention? Let me know so we can coordinate the change.
There's a new function subContextFromGenotypes() that also takes a Set<Allele> to handle this type of behavior.
git-svn-id: file:///humgen/gsa-scr1/gsa-engineering/svn_contents/trunk@5832 348d0f76-0448-11de-a6fe-93d51630548a
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