How to interpret genotypes with DP=1 for a vcf file

Question

[zhangdezhi008]

Dear all, I used samtools for SNPs calling and vcftools for SNPs filtering. I got a vcf file with a lot SNPs.How to interpret the genotypes when DP=1? In my opinion, DP=1 means that this site has only one read, it could be homozygous 0/0 or 1/1, but how can it be a heterozygous 0/1?

The following is what I have observed in my vcf file.

Thanks for your attentions! Please help!

GT:PL:DP:SP:GQ 0/0:0,3,36:1:0:4

GT:PL:DP:SP:GQ 0/1:0,3,36:1:0:4

GT:PL:DP:SP:GQ 1/1:0,3,36:1:0:4

[Benn]

It can be heterozygous if you find one read with the alternative base (instead of the reference base). But do you trust these DP=1 calls? I mean 1 read is pretty minimal.

Edit: I see your point, you mean with one read of the alt allele it could be both homozygous or heterozygous.

[zhangdezhi008]

Thanks for your reply. I will not trust the lower DP SNPs. How can one read possesses an alternative base? I also have another question, how can we know the depth for each allele for a heterozygous site?

[Benn]

I am not sure about your method, I never used vcftools for this. When using varscan after samtools, I get more info than you get: e.g.,

GT:GQ:SDP:DP:RD:AD:FREQ:PVAL:RBQ:ABQ:RDF:RDR:ADF:ADR
1/1:117:31:21:0:21:100%:1.8578E-12:0:25:0:0:5:16

With other meaning for DP as well:

##FORMAT=<ID=GT,Number=1,Type=String,Description="Genotype">
##FORMAT=<ID=GQ,Number=1,Type=Integer,Description="Genotype Quality">
##FORMAT=<ID=SDP,Number=1,Type=Integer,Description="Raw Read Depth as reported by SAMtools">
##FORMAT=<ID=DP,Number=1,Type=Integer,Description="Quality Read Depth of bases with Phred score >= 15">
##FORMAT=<ID=RD,Number=1,Type=Integer,Description="Depth of reference-supporting bases (reads1)">
##FORMAT=<ID=AD,Number=1,Type=Integer,Description="Depth of variant-supporting bases (reads2)">
##FORMAT=<ID=FREQ,Number=1,Type=String,Description="Variant allele frequency">
##FORMAT=<ID=PVAL,Number=1,Type=String,Description="P-value from Fisher's Exact Test">
##FORMAT=<ID=RBQ,Number=1,Type=Integer,Description="Average quality of reference-supporting bases (qual1)">
##FORMAT=<ID=ABQ,Number=1,Type=Integer,Description="Average quality of variant-supporting bases (qual2)">
##FORMAT=<ID=RDF,Number=1,Type=Integer,Description="Depth of reference-supporting bases on forward strand (reads1plus)">
##FORMAT=<ID=RDR,Number=1,Type=Integer,Description="Depth of reference-supporting bases on reverse strand (reads1minus)">
##FORMAT=<ID=ADF,Number=1,Type=Integer,Description="Depth of variant-supporting bases on forward strand (reads2plus)">
##FORMAT=<ID=ADR,Number=1,Type=Integer,Description="Depth of variant-supporting bases on reverse strand (reads2minus)">

[stolarek.ir]

DP4 field has information about reads that support: reference positive strand, reference negative, alternative positive, alternative negative

But check out documentation if it's in that order