Strange GT and GQ when using sample_ploidy set to 1
Posted in Ask the GATK team | Last updated on 2012-10-18 01:03:55

Comments (6)

I am experiencing a problem with the ploidy 1 option. Having used GATK2 unified genotyper with the params --sample_ploidy 1 --genotype_likelihoods_model BOTH -rf BadCigar I get the following line in a vcf file (see sample in bold)

Staphylococcus 1553115 . A G 24454.01 . AC=13;AF=0.813;AN=16;BaseQRankSum=1.072;DP=1040;Dels=0.00;FS=32.822;HaplotypeScore=3.3463;MLEAC=13;MLEAF=0.813;MQ=40.20;MQ0=47;MQRankSum=-10.543;QD=32.13;ReadPosRankSum=-1.148;SB=-9.076e+03 GT:AD:DP:GQ:MLPSAC:MLPSAF:PL 1:0,29:29:99:1:1.00:1015,0 1:0,62:62:99:1:1.00:2053,0 1:0,106:106:99:1:1.00:3210,0 1:0,102:102:99:1:1.00:3305,0 1:0,88:88:99:1:1.00:2750,0 1:0,41:41:99:1:1.00:1324,0 1:0,76:76:99:1:1.00:2448,0 1:0,39:39:99:1:1.00:1303,0 0:64,40:104:99:0:0.00:0,1334 1:0,41:41:99:1:1.00:1373,0 1:0,49:49:99:1:1.00:1668,0 0:72,50:122:99:0:0.00:0,1258 1:0,59:59:99:1:1.00:1852,0 1:0,38:38:99:1:1.00:1192,0 1:0,31:31:99:1:1.00:961,0 0:53,0:53:99:0:0.00:0,1633

The sample in bold is called as WT (genotype 0) with a high GQ despite there being 72 reads of genotype 0 and 50 of genotype 1. Examining the bam file suggests that this is a mapping error in a repetitive phage region

If I set ploidy to be 2 the equivalent line in the resulting vcf file is

Staphylococcus 1553115 . A G 24788.02 . AC=28;AF=0.875;AN=32;BaseQRankSum=0.947;DP=1040;Dels=0.00;FS=32.822;HaplotypeScore=3.3463;InbreedingCoeff=0.4286;MLEAC=28;MLEAF=0.875;MQ=40.20;MQ0=47;MQRankSum=-10.096;QD=25.11;ReadPosRankSum=-1.177;SB=-9.871e+03 GT:AD:DP:GQ:PL 1/1:0,29:29:81:986,81,0 1/1:0,62:62:99:1895,156,0 1/1:0,106:106:99:2992,247,0 1/1:0,102:102:99:3169,268,0 1/1:0,88:88:99:2452,193,0 1/1:0,41:41:99:1243,99,0 1/1:0,76:76:99:2283,193,0 1/1:0,39:39:99:1233,105,0 0/1:64,40:104:99:886,0,1706 1/1:0,41:41:99:1298,108,0 1/1:0,49:49:99:1581,129,0 0/1:72,50:122:99:1235,0,2126 1/1:0,59:59:99:1649,132,0 1/1:0,38:38:87:1065,87,0 1/1:0,31:31:69:821,69,0 0/0:53,0:53:99:0,138,1588

As can be seen from the bold text, the same position is called as heterozygote which based on the number of the reads mapping would be likley except for the fact this is a bacterial haploid genome. Previously I would have discarded this since the heterozygous call indicates mis-mapping as the bam file confirms. I had been hoping to use the sample_polidy option set to 1 for bacterial genomes but this result concerns me. I could obviously filter based on AD but the wonder why the sample was given a high GQ when the polidy is set to 1 and the AD suggests the call of genotype 0 should be doubted.

Any suggestions on what is going on here?? Many thanks



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