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VCF Manipulation

Merge, concat, sort, and compare VCF files using bcftools.

Operations Overview

| Operation | Command | Use Case | |-----------|---------|----------| | Merge | bcftools merge | Combine samples from multiple VCFs | | Concat | bcftools concat | Combine regions from multiple VCFs | | Sort | bcftools sort | Sort unsorted VCF | | Intersect | bcftools isec | Compare/intersect call sets | | Subset | bcftools view | Extract samples or regions |

bcftools merge

Combine multiple VCF files with different samples at the same positions.

Basic Merge

bash
bcftools merge sample1.vcf.gz sample2.vcf.gz -Oz -o merged.vcf.gz

Merge Multiple Files

bash
bcftools merge *.vcf.gz -Oz -o all_samples.vcf.gz

Merge from File List

bash
# files.txt: one VCF path per line
bcftools merge -l files.txt -Oz -o merged.vcf.gz

Handle Missing Genotypes

bash
# Output missing genotypes as ./. (default)
bcftools merge sample1.vcf.gz sample2.vcf.gz -Oz -o merged.vcf.gz

# Output missing as reference (0/0)
bcftools merge --missing-to-ref sample1.vcf.gz sample2.vcf.gz -Oz -o merged.vcf.gz

Force Sample Names

When sample names conflict:

bash
bcftools merge --force-samples sample1.vcf.gz sample2.vcf.gz -Oz -o merged.vcf.gz

Merge Specific Regions

bash
bcftools merge -r chr1:1000000-2000000 sample1.vcf.gz sample2.vcf.gz -Oz -o merged.vcf.gz

bcftools concat

Combine VCF files with same samples from different regions.

Concatenate Chromosomes

bash
bcftools concat chr1.vcf.gz chr2.vcf.gz chr3.vcf.gz -Oz -o genome.vcf.gz

Concatenate All Chromosomes

bash
bcftools concat chr*.vcf.gz -Oz -o genome.vcf.gz

From File List

bash
# files.txt: one VCF path per line (in order)
bcftools concat -f files.txt -Oz -o concatenated.vcf.gz

Allow Overlapping Regions

bash
bcftools concat -a chr1_part1.vcf.gz chr1_part2.vcf.gz -Oz -o chr1.vcf.gz

Remove Duplicates

bash
bcftools concat -a -d all file1.vcf.gz file2.vcf.gz -Oz -o merged.vcf.gz

Options for -d:

• snps - Remove duplicate SNPs
• indels - Remove duplicate indels
• both - Remove duplicate SNPs and indels
• all - Remove all duplicates
• exact - Remove exact duplicates only

bcftools sort

Sort VCF by chromosome and position.

Basic Sort

bash
bcftools sort input.vcf -Oz -o sorted.vcf.gz

With Temporary Directory

For large files:

bash
bcftools sort -T /tmp input.vcf.gz -Oz -o sorted.vcf.gz

Memory Limit

bash
bcftools sort -m 4G input.vcf.gz -Oz -o sorted.vcf.gz

bcftools isec

Intersect and compare VCF files.

Find Shared Variants

bash
bcftools isec -p output_dir sample1.vcf.gz sample2.vcf.gz

Creates:

• 0000.vcf - Private to sample1
• 0001.vcf - Private to sample2
• 0002.vcf - Shared (sample1 records)
• 0003.vcf - Shared (sample2 records)

Output Compressed

bash
bcftools isec -p output_dir -Oz sample1.vcf.gz sample2.vcf.gz

Intersection Only

bash
bcftools isec -p output_dir -n=2 sample1.vcf.gz sample2.vcf.gz
# Only outputs variants present in exactly 2 files

Comparison Options

| Flag | Description | |------|-------------| | -n=2 | Present in exactly 2 files | | -n+2 | Present in 2 or more files | | -n-2 | Present in fewer than 2 files | | -n~11 | Boolean: file1 AND file2 | | -n~10 | Boolean: file1 AND NOT file2 |

Two-File Intersection

bash
# Variants in both files
bcftools isec -n=2 -w1 sample1.vcf.gz sample2.vcf.gz -Oz -o shared.vcf.gz

# Variants only in sample1
bcftools isec -n~10 -w1 sample1.vcf.gz sample2.vcf.gz -Oz -o only_sample1.vcf.gz

Complement Mode

bash
# Variants in file1 not in file2
bcftools isec -C sample1.vcf.gz sample2.vcf.gz -Oz -o unique.vcf.gz

Subsetting VCF Files

Extract Samples

bash
bcftools view -s sample1,sample2 input.vcf.gz -Oz -o subset.vcf.gz

Exclude Samples

bash
bcftools view -s ^sample3 input.vcf.gz -Oz -o without_sample3.vcf.gz

From Sample List File

bash
# samples.txt: one sample name per line
bcftools view -S samples.txt input.vcf.gz -Oz -o subset.vcf.gz

Extract Region

bash
bcftools view -r chr1:1000000-2000000 input.vcf.gz -Oz -o region.vcf.gz

Extract Multiple Regions

bash
bcftools view -R regions.bed input.vcf.gz -Oz -o targets.vcf.gz

Renaming Samples

Single Sample

bash
echo "old_name new_name" > rename.txt
bcftools reheader -s rename.txt input.vcf.gz -o renamed.vcf.gz

Multiple Samples

bash
# rename.txt format: old_name new_name
cat > rename.txt << EOF
sample1 patient_001
sample2 patient_002
sample3 patient_003
EOF

bcftools reheader -s rename.txt input.vcf.gz -o renamed.vcf.gz

Splitting VCF Files

Split by Sample

bash
for sample in $(bcftools query -l input.vcf.gz); do
    bcftools view -s "$sample" input.vcf.gz -Oz -o "${sample}.vcf.gz"
done

Split by Chromosome

bash
for chr in $(bcftools view -h input.vcf.gz | grep "^##contig" | sed 's/.*ID=\([^,]*\).*/\1/'); do
    bcftools view -r "$chr" input.vcf.gz -Oz -o "${chr}.vcf.gz"
done

Split Multiallelic Sites

bash
bcftools norm -m-any input.vcf.gz -Oz -o split.vcf.gz

Common Workflows

Merge Cohort VCFs

bash
# Create file list
ls *.vcf.gz > files.txt

# Merge all samples
bcftools merge -l files.txt -Oz -o cohort.vcf.gz
bcftools index cohort.vcf.gz

Combine Chromosome VCFs

bash
# After parallel variant calling by chromosome
bcftools concat chr{1..22}.vcf.gz chrX.vcf.gz chrY.vcf.gz -Oz -o genome.vcf.gz
bcftools index genome.vcf.gz

Compare Two Callers

bash
# Find variants called by both GATK and bcftools
bcftools isec -p comparison gatk.vcf.gz bcftools.vcf.gz

# Count results
wc -l comparison/*.vcf

Extract Passing Variants

bash
bcftools view -f PASS input.vcf.gz -Oz -o pass_only.vcf.gz
bcftools index pass_only.vcf.gz

cyvcf2 Python Operations

Note: True VCF merging (combining samples at matching positions) is complex. Use bcftools merge for production work. cyvcf2 is better for filtering/querying.

Concatenate Records (Not True Merge)

python
from cyvcf2 import VCF, Writer

# WARNING: This concatenates records, not a true merge
# For actual merging of samples, use bcftools merge
vcf1 = VCF('file1.vcf.gz')
writer = Writer('combined.vcf', vcf1)

for variant in vcf1:
    writer.write_record(variant)

writer.close()
vcf1.close()

Find Shared Positions

python
from cyvcf2 import VCF

# Load positions from first VCF
vcf1_positions = set()
for variant in VCF('sample1.vcf.gz'):
    vcf1_positions.add((variant.CHROM, variant.POS))

# Check second VCF
shared = 0
unique = 0
for variant in VCF('sample2.vcf.gz'):
    if (variant.CHROM, variant.POS) in vcf1_positions:
        shared += 1
    else:
        unique += 1

print(f'Shared: {shared}')
print(f'Unique to sample2: {unique}')

Quick Reference

| Task | Command | |------|---------| | Merge samples | bcftools merge s1.vcf.gz s2.vcf.gz -Oz -o merged.vcf.gz | | Concat regions | bcftools concat chr1.vcf.gz chr2.vcf.gz -Oz -o all.vcf.gz | | Sort VCF | bcftools sort input.vcf -Oz -o sorted.vcf.gz | | Intersect | bcftools isec -p dir a.vcf.gz b.vcf.gz | | Extract samples | bcftools view -s sample1 input.vcf.gz | | Rename samples | bcftools reheader -s names.txt input.vcf.gz |

Common Errors

| Error | Cause | Solution | |-------|-------|----------| | different samples | merge vs concat confusion | Use merge for samples, concat for regions | | not sorted | Unsorted input to concat | Sort first or use -a flag | | sample name conflict | Duplicate sample names | Use --force-samples | | index required | Missing index for merge/isec | Run bcftools index first |

Related Skills

• vcf-basics - View and query VCF files
• filtering-best-practices - Filter variants before manipulation
• variant-normalization - Normalize before comparing
• vcf-statistics - Compare statistics after manipulation

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