Vcf Basics

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---
name: bio-vcf-basics
description: View, query, and understand VCF/BCF variant files using bcftools and cyvcf2. Use when inspecting variants, extracting specific fields, or understanding VCF format structure.
tool_type: cli
primary_tool: bcftools
measurable_outcome: Execute skill workflow successfully with valid output within 15 minutes.
allowed-tools:
  - read_file
  - run_shell_command
---

# VCF/BCF Basics

View and query variant files using bcftools and cyvcf2.

## Format Overview

| Format | Description | Use Case |
|--------|-------------|----------|
| VCF | Text format, human-readable | Debugging, small files |
| VCF.gz | Compressed VCF (bgzip) | Standard distribution |
| BCF | Binary VCF | Fast processing, large files |

## VCF Format Structure

```
##fileformat=VCFv4.2
##INFO=<ID=DP,Number=1,Type=Integer,Description="Total Depth">
##FORMAT=<ID=GT,Number=1,Type=String,Description="Genotype">
##FORMAT=<ID=DP,Number=1,Type=Integer,Description="Read Depth">
#CHROM  POS     ID      REF     ALT     QUAL    FILTER  INFO    FORMAT  SAMPLE1
chr1    1000    rs123   A       G       30      PASS    DP=50   GT:DP   0/1:25
```

### Header Lines (##)
- `##fileformat` - VCF version
- `##INFO` - INFO field definitions
- `##FORMAT` - FORMAT field definitions
- `##FILTER` - Filter definitions
- `##contig` - Reference contigs
- `##reference` - Reference genome

### Column Header (#CHROM)
Fixed columns: CHROM, POS, ID, REF, ALT, QUAL, FILTER, INFO, FORMAT
Followed by sample columns

### Data Columns

| Column | Description |
|--------|-------------|
| CHROM | Chromosome |
| POS | 1-based position |
| ID | Variant identifier (e.g., rs number) |
| REF | Reference allele |
| ALT | Alternate allele(s), comma-separated |
| QUAL | Phred-scaled quality score |
| FILTER | PASS or filter name |
| INFO | Semicolon-separated key=value pairs |
| FORMAT | Colon-separated format keys |
| SAMPLE | Colon-separated values matching FORMAT |

## bcftools view

### View VCF
```bash
bcftools view input.vcf.gz | head
```

### View Header Only
```bash
bcftools view -h input.vcf.gz
```

### View Without Header
```bash
bcftools view -H input.vcf.gz | head
```

### View Specific Region
```bash
bcftools view input.vcf.gz chr1:1000000-2000000
```

### View Specific Samples
```bash
bcftools view -s sample1,sample2 input.vcf.gz
```

### Exclude Samples
```bash
bcftools view -s ^sample3 input.vcf.gz
```

## bcftools query

Extract specific fields in custom format.

### Basic Query
```bash
bcftools query -f '%CHROM\t%POS\t%REF\t%ALT\n' input.vcf.gz
```

### Query with INFO Fields
```bash
bcftools query -f '%CHROM\t%POS\t%INFO/DP\t%INFO/AF\n' input.vcf.gz
```

### Query with Sample Fields
```bash
bcftools query -f '%CHROM\t%POS[\t%GT]\n' input.vcf.gz
```

### Query Specific Samples
```bash
bcftools query -f '%CHROM\t%POS[\t%SAMPLE=%GT]\n' -s sample1,sample2 input.vcf.gz
```

### Include Header
```bash
bcftools query -H -f '%CHROM\t%POS\t%REF\t%ALT\n' input.vcf.gz
```

### Common Format Specifiers

| Specifier | Description |
|-----------|-------------|
| `%CHROM` | Chromosome |
| `%POS` | Position |
| `%ID` | Variant ID |
| `%REF` | Reference allele |
| `%ALT` | Alternate allele |
| `%QUAL` | Quality score |
| `%FILTER` | Filter status |
| `%INFO/TAG` | INFO field value |
| `%TYPE` | Variant type (snp, indel, etc.) |
| `[%GT]` | Genotype (per sample) |
| `[%DP]` | Depth (per sample) |
| `[%SAMPLE]` | Sample name |
| `\n` | Newline |
| `\t` | Tab |

## Format Conversion

### VCF to BCF
```bash
bcftools view -Ob -o output.bcf input.vcf.gz
```

### BCF to VCF
```bash
bcftools view -Ov -o output.vcf input.bcf
```

### Compress VCF (bgzip)
```bash
bgzip input.vcf
# Creates input.vcf.gz
```

### Index VCF/BCF
```bash
bcftools index input.vcf.gz
# Creates input.vcf.gz.csi

bcftools index -t input.vcf.gz
# Creates input.vcf.gz.tbi (tabix index)
```

## Output Format Options

| Flag | Format |
|------|--------|
| `-Ov` | Uncompressed VCF |
| `-Oz` | Compressed VCF (bgzip) |
| `-Ou` | Uncompressed BCF |
| `-Ob` | Compressed BCF |

## Genotype Encoding

| Genotype | Meaning |
|----------|---------|
| `0/0` | Homozygous reference |
| `0/1` | Heterozygous |
| `1/1` | Homozygous alternate |
| `1/2` | Heterozygous (two different alts) |
| `./.` | Missing |
| `0\|1` | Phased heterozygous |

## cyvcf2 Python Alternative

### Open and Iterate
```python
from cyvcf2 import VCF

vcf = VCF('input.vcf.gz')
for variant in vcf:
    print(f'{variant.CHROM}:{variant.POS} {variant.REF}>{variant.ALT[0]}')
```

### Access Variant Properties
```python
from cyvcf2 import VCF

for variant in VCF('input.vcf.gz'):
    print(f'Chrom: {variant.CHROM}')
    print(f'Pos: {variant.POS}')
    print(f'ID: {variant.ID}')
    print(f'Ref: {variant.REF}')
    print(f'Alt: {variant.ALT}')  # List
    print(f'Qual: {variant.QUAL}')
    print(f'Filter: {variant.FILTER}')
    print(f'Type: {variant.var_type}')  # snp, indel, etc.
    break
```

### Access INFO Fields
```python
from cyvcf2 import VCF

for variant in VCF('input.vcf.gz'):
    dp = variant.INFO.get('DP')
    af = variant.INFO.get('AF')
    print(f'{variant.CHROM}:{variant.POS} DP={dp} AF={af}')
```

### Access Genotypes
```python
from cyvcf2 import VCF

vcf = VCF('input.vcf.gz')
samples = vcf.samples  # List of sample names

for variant in vcf:
    gts = variant.gt_types  # 0=HOM_REF, 1=HET, 2=UNKNOWN, 3=HOM_ALT
    for sample, gt in zip(samples, gts):
        gt_str = ['HOM_REF', 'HET', 'UNKNOWN', 'HOM_ALT'][gt]
        print(f'{sample}: {gt_str}')
    break
```

### Access Sample Fields
```python
from cyvcf2 import VCF

for variant in VCF('input.vcf.gz'):
    depths = variant.format('DP')  # numpy array
    gqs = variant.format('GQ')     # Genotype quality
    print(f'Depths: {depths}')
```

### Fetch Region
```python
from cyvcf2 import VCF

vcf = VCF('input.vcf.gz')
for variant in vcf('chr1:1000000-2000000'):
    print(f'{variant.CHROM}:{variant.POS}')
```

### Get Header Info
```python
from cyvcf2 import VCF

vcf = VCF('input.vcf.gz')
print(f'Samples: {vcf.samples}')
print(f'Contigs: {vcf.seqnames}')

# INFO field definitions
for info in vcf.header_iter():
    if info['HeaderType'] == 'INFO':
        print(f'{info["ID"]}: {info["Description"]}')
```

### Write VCF
```python
from cyvcf2 import VCF, Writer

vcf = VCF('input.vcf.gz')
writer = Writer('output.vcf', vcf)

for variant in vcf:
    if variant.QUAL > 30:
        writer.write_record(variant)

writer.close()
vcf.close()
```

## Quick Reference

| Task | bcftools | cyvcf2 |
|------|----------|--------|
| View VCF | `bcftools view file.vcf.gz` | `VCF('file.vcf.gz')` |
| View header | `bcftools view -h file.vcf.gz` | `vcf.header_iter()` |
| Get region | `bcftools view file.vcf.gz chr1:1-1000` | `vcf('chr1:1-1000')` |
| Query fields | `bcftools query -f '%CHROM\t%POS\n'` | Loop with properties |
| Count variants | `bcftools view -H file.vcf.gz \| wc -l` | `sum(1 for _ in vcf)` |
| VCF to BCF | `bcftools view -Ob -o out.bcf in.vcf.gz` | Use Writer |

## Common Errors

| Error | Cause | Solution |
|-------|-------|----------|
| `no BGZF EOF marker` | Not bgzipped | Use `bgzip` not `gzip` |
| `index required` | Missing index for region query | Run `bcftools index` |
| `sample not found` | Wrong sample name | Check with `bcftools query -l` |

## Related Skills

- variant-calling - Generate VCF from alignments
- filtering-best-practices - Filter variants by quality/criteria
- vcf-manipulation - Merge, concat, intersect VCFs
- alignment-files/pileup-generation - Generate pileup for calling


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