/exploratory-data-analysis

Bioinformatics and Genomics File Formats Reference

This reference covers file formats used in genomics, transcriptomics, sequence analysis, and related bioinformatics applications.

Sequence Data Formats

.fasta / .fa / .fna - FASTA Format

Description: Text-based format for nucleotide or protein sequences Typical Data: DNA, RNA, or protein sequences with headers Use Cases: Sequence storage, BLAST searches, alignments Python Libraries:

• Biopython: SeqIO.parse('file.fasta', 'fasta')
• pyfaidx: Fast indexed FASTA access
• screed: Fast sequence parsing EDA Approach:
• Sequence count and length distribution
• GC content analysis
• N content (ambiguous bases)
• Sequence ID parsing
• Duplicate detection
• Quality metrics for assemblies (N50, L50)

.fastq / .fq - FASTQ Format

Description: Sequence data with base quality scores Typical Data: Raw sequencing reads with Phred quality scores Use Cases: NGS data, quality control, read mapping Python Libraries:

• Biopython: SeqIO.parse('file.fastq', 'fastq')
• pysam: Fast FASTQ/BAM operations
• HTSeq: Sequencing data analysis EDA Approach:
• Read count and length distribution
• Quality score distribution (per-base, per-read)
• GC content and bias
• Duplicate rate estimation
• Adapter contamination detection
• k-mer frequency analysis
• Encoding format validation (Phred33/64)

.sam - Sequence Alignment/Map

Description: Tab-delimited text format for alignments Typical Data: Aligned sequencing reads with mapping quality Use Cases: Read alignment storage, variant calling Python Libraries:

• pysam: pysam.AlignmentFile('file.sam', 'r')
• HTSeq: HTSeq.SAM_Reader('file.sam') EDA Approach:
• Mapping rate and quality distribution
• Coverage analysis
• Insert size distribution (paired-end)
• Alignment flags distribution
• CIGAR string patterns
• Mismatch and indel rates
• Duplicate and supplementary alignment counts

.bam - Binary Alignment/Map

Description: Compressed binary version of SAM Typical Data: Aligned reads in compressed format Use Cases: Efficient storage and processing of alignments Python Libraries:

• pysam: Full BAM support with indexing
• bamnostic: Pure Python BAM reader EDA Approach:
• Same as SAM plus:
• Compression ratio analysis
• Index file (.bai) validation
• Chromosome-wise statistics
• Strand bias detection
• Read group analysis

.cram - CRAM Format

Description: Highly compressed alignment format Typical Data: Reference-compressed aligned reads Use Cases: Long-term storage, space-efficient archives Python Libraries:

• pysam: CRAM support (requires reference)
• Reference genome must be accessible EDA Approach:
• Compression efficiency vs BAM
• Reference dependency validation
• Lossy vs lossless compression assessment
• Decompression performance
• Similar alignment metrics as BAM

.bed - Browser Extensible Data

Description: Tab-delimited format for genomic features Typical Data: Genomic intervals (chr, start, end) with annotations Use Cases: Peak calling, variant annotation, genome browsing Python Libraries:

• pybedtools: pybedtools.BedTool('file.bed')
• pyranges: pyranges.read_bed('file.bed')
• pandas: Simple BED reading EDA Approach:
• Feature count and size distribution
• Chromosome distribution
• Strand bias
• Score distribution (if present)
• Overlap and proximity analysis
• Coverage statistics
• Gap analysis between features

.bedGraph - BED with Graph Data

Description: BED format with per-base signal values Typical Data: Continuous-valued genomic data (coverage, signals) Use Cases: Coverage tracks, ChIP-seq signals, methylation Python Libraries:

• pyBigWig: Can convert to bigWig
• pybedtools: BedGraph operations EDA Approach:
• Signal distribution statistics
• Genome coverage percentage
• Signal dynamics (peaks, valleys)
• Chromosome-wise signal patterns
• Quantile analysis
• Zero-coverage regions

.bigWig / .bw - Binary BigWig

Description: Indexed binary format for genome-wide signal data Typical Data: Continuous genomic signals (compressed and indexed) Use Cases: Efficient genome browser tracks, large-scale data Python Libraries:

• pyBigWig: pyBigWig.open('file.bw')
• pybbi: BigWig/BigBed interface EDA Approach:
• Signal statistics extraction
• Zoom level analysis
• Regional signal extraction
• Efficient genome-wide summaries
• Compression efficiency
• Index structure analysis

.bigBed / .bb - Binary BigBed

Description: Indexed binary BED format Typical Data: Genomic features (compressed and indexed) Use Cases: Large feature sets, genome browsers Python Libraries:

• pybbi: BigBed reading
• pybigtools: Modern BigBed interface EDA Approach:
• Feature density analysis
• Efficient interval queries
• Zoom level validation
• Index performance metrics
• Feature size statistics

.gff / .gff3 - General Feature Format

Description: Tab-delimited format for genomic annotations Typical Data: Gene models, transcripts, exons, regulatory elements Use Cases: Genome annotation, gene prediction Python Libraries:

• BCBio.GFF: Biopython GFF module
• gffutils: gffutils.create_db('file.gff3')
• pyranges: GFF support EDA Approach:
• Feature type distribution (gene, exon, CDS, etc.)
• Gene structure validation
• Strand balance
• Hierarchical relationship validation
• Phase validation for CDS
• Attribute completeness
• Gene model statistics (introns, exons per gene)

.gtf - Gene Transfer Format

Description: GFF2-based format for gene annotations Typical Data: Gene and transcript annotations Use Cases: RNA-seq analysis, gene quantification Python Libraries:

• pyranges: pyranges.read_gtf('file.gtf')
• gffutils: GTF database creation
• HTSeq: GTF reading for counts EDA Approach:
• Transcript isoform analysis
• Gene structure completeness
• Exon number distribution
• Transcript length distribution
• TSS and TES analysis
• Biotype distribution
• Overlapping gene detection

.vcf - Variant Call Format

Description: Text format for genetic variants Typical Data: SNPs, indels, structural variants with annotations Use Cases: Variant calling, population genetics, GWAS Python Libraries:

• pysam: pysam.VariantFile('file.vcf')
• cyvcf2: Fast VCF parsing
• PyVCF: Older but comprehensive EDA Approach:
• Variant count by type (SNP, indel, SV)
• Quality score distribution
• Allele frequency spectrum
• Transition/transversion ratio
• Heterozygosity rates
• Missing genotype analysis
• Hardy-Weinberg equilibrium
• Annotation completeness (if annotated)

.bcf - Binary VCF

Description: Compressed binary variant format Typical Data: Same as VCF but binary Use Cases: Efficient variant storage and processing Python Libraries:

• pysam: Full BCF support
• cyvcf2: Optimized BCF reading EDA Approach:
• Same as VCF plus:
• Compression efficiency
• Indexing validation
• Read performance metrics

.gvcf - Genomic VCF

Description: VCF with reference confidence blocks Typical Data: All positions (variant and non-variant) Use Cases: Joint genotyping workflows, GATK Python Libraries:

• pysam: GVCF support
• Standard VCF parsers EDA Approach:
• Reference block analysis
• Coverage uniformity
• Variant density
• Genotype quality across genome
• Reference confidence distribution

RNA-Seq and Expression Data

.counts - Gene Count Matrix

Description: Tab-delimited gene expression counts Typical Data: Gene IDs with read counts per sample Use Cases: RNA-seq quantification, differential expression Python Libraries:

• pandas: pd.read_csv('file.counts', sep='\t')
• scanpy (for single-cell): sc.read_csv() EDA Approach:
• Library size distribution
• Detection rate (genes per sample)
• Zero-inflation analysis
• Count distribution (log scale)
• Outlier sample detection
• Correlation between replicates
• PCA for sample relationships

.tpm / .fpkm - Normalized Expression

Description: Normalized gene expression values Typical Data: TPM (transcripts per million) or FPKM values Use Cases: Cross-sample comparison, visualization Python Libraries:

• pandas: Standard CSV reading
• anndata: For integrated analysis EDA Approach:
• Expression distribution
• Highly expressed gene identification
• Sample clustering
• Batch effect detection
• Coefficient of variation analysis
• Dynamic range assessment

.mtx - Matrix Market Format

Description: Sparse matrix format (common in single-cell) Typical Data: Sparse count matrices (cells × genes) Use Cases: Single-cell RNA-seq, large sparse matrices Python Libraries:

• scipy.io: scipy.io.mmread('file.mtx')
• scanpy: sc.read_mtx('file.mtx') EDA Approach:
• Sparsity analysis
• Cell and gene filtering thresholds
• Doublet detection metrics
• Mitochondrial fraction
• UMI count distribution
• Gene detection per cell

.h5ad - Anndata Format

Description: HDF5-based annotated data matrix Typical Data: Expression matrix with metadata (cells, genes) Use Cases: Single-cell RNA-seq analysis with Scanpy Python Libraries:

• scanpy: sc.read_h5ad('file.h5ad')
• anndata: Direct AnnData manipulation EDA Approach:
• Cell and gene counts
• Metadata completeness
• Layer availability (raw, normalized)
• Embedding presence (PCA, UMAP)
• QC metrics distribution
• Batch information
• Cell type annotation coverage

.loom - Loom Format

Description: HDF5-based format for omics data Typical Data: Expression matrices with metadata Use Cases: Single-cell data, RNA velocity analysis Python Libraries:

• loompy: loompy.connect('file.loom')
• scanpy: Can import loom files EDA Approach:
• Layer analysis (spliced, unspliced)
• Row and column attribute exploration
• Graph connectivity analysis
• Cluster assignments
• Velocity-specific metrics

.rds - R Data Serialization

Description: R object storage (often Seurat objects) Typical Data: R analysis results, especially single-cell Use Cases: R-Python data exchange Python Libraries:

• pyreadr: pyreadr.read_r('file.rds')
• rpy2: For full R integration
• Conversion tools to AnnData EDA Approach:
• Object type identification
• Data structure exploration
• Metadata extraction
• Conversion validation

Alignment and Assembly Formats

.maf - Multiple Alignment Format

Description: Text format for multiple sequence alignments Typical Data: Genome-wide or local multiple alignments Use Cases: Comparative genomics, conservation analysis Python Libraries:

• Biopython: AlignIO.parse('file.maf', 'maf')
• bx-python: MAF-specific tools EDA Approach:
• Alignment block statistics
• Species coverage
• Gap analysis
• Conservation scoring
• Alignment quality metrics
• Block length distribution

.axt - Pairwise Alignment Format

Description: Pairwise alignment format (UCSC) Typical Data: Pairwise genomic alignments Use Cases: Genome comparison, synteny analysis Python Libraries:

• Custom parsers (simple format)
• bx-python: AXT support EDA Approach:
• Alignment score distribution
• Identity percentage
• Syntenic block identification
• Gap size analysis
• Coverage statistics

.chain - Chain Alignment Format

Description: Genome coordinate mapping chains Typical Data: Coordinate transformations between genome builds Use Cases: Liftover, coordinate conversion Python Libraries:

• pyliftover: Chain file usage
• Custom parsers for chain format EDA Approach:
• Chain score distribution
• Coverage of source genome
• Gap analysis
• Inversion detection
• Mapping quality assessment

.psl - Pattern Space Layout

Description: BLAT/BLAST alignment format Typical Data: Alignment results from BLAT Use Cases: Transcript mapping, similarity searches Python Libraries:

• Custom parsers (tab-delimited)
• pybedtools: Can handle PSL EDA Approach:
• Match percentage distribution
• Gap statistics
• Query coverage
• Multiple mapping analysis
• Alignment quality metrics

Genome Assembly and Annotation

.agp - Assembly Golden Path

Description: Assembly structure description Typical Data: Scaffold composition, gap information Use Cases: Genome assembly representation Python Libraries:

• Custom parsers (simple tab-delimited)
• Assembly analysis tools EDA Approach:
• Scaffold statistics (N50, L50)
• Gap type and size distribution
• Component length analysis
• Assembly contiguity metrics
• Unplaced contig analysis

.scaffolds / .contigs - Assembly Sequences

Description: Assembled sequences (usually FASTA) Typical Data: Assembled genomic sequences Use Cases: Genome assembly output Python Libraries:

• Same as FASTA format
• Assembly-specific tools (QUAST) EDA Approach:
• Assembly statistics (N50, N90, etc.)
• Length distribution
• Coverage analysis
• Gap (N) content
• Duplication assessment
• BUSCO completeness (if annotations available)

.2bit - Compressed Genome Format

Description: UCSC compact genome format Typical Data: Reference genomes (highly compressed) Use Cases: Efficient genome storage and access Python Libraries:

• py2bit: py2bit.open('file.2bit')
• twobitreader: Alternative reader EDA Approach:
• Compression efficiency
• Random access performance
• Sequence extraction validation
• Masked region analysis
• N content and distribution

.sizes - Chromosome Sizes

Description: Simple format with chromosome lengths Typical Data: Tab-delimited chromosome names and sizes Use Cases: Genome browsers, coordinate validation Python Libraries:

• Simple file reading with pandas
• Built into many genomic tools EDA Approach:
• Genome size calculation
• Chromosome count
• Size distribution
• Karyotype validation
• Completeness check against reference

Phylogenetics and Evolution

.nwk / .newick - Newick Tree Format

Description: Parenthetical tree representation Typical Data: Phylogenetic trees with branch lengths Use Cases: Evolutionary analysis, tree visualization Python Libraries:

• Biopython: Phylo.read('file.nwk', 'newick')
• ete3: ete3.Tree('file.nwk')
• dendropy: Phylogenetic computing EDA Approach:
• Tree structure analysis (tips, internal nodes)
• Branch length distribution
• Tree balance metrics
• Ultrametricity check
• Bootstrap support analysis
• Topology validation

.nexus - Nexus Format

Description: Rich format for phylogenetic data Typical Data: Alignments, trees, character matrices Use Cases: Phylogenetic software interchange Python Libraries:

• Biopython: Nexus support
• dendropy: Comprehensive Nexus handling EDA Approach:
• Data block analysis
• Character type distribution
• Tree block validation
• Taxa consistency
• Command block parsing
• Format compliance checking

.phylip - PHYLIP Format

Description: Sequence alignment format (strict/relaxed) Typical Data: Multiple sequence alignments Use Cases: Phylogenetic analysis input Python Libraries:

• Biopython: AlignIO.read('file.phy', 'phylip')
• dendropy: PHYLIP support EDA Approach:
• Alignment dimensions
• Sequence length uniformity
• Gap position analysis
• Informative site calculation
• Format variant detection (strict vs relaxed)

.paml - PAML Output

Description: Output from PAML phylogenetic software Typical Data: Evolutionary model results, dN/dS ratios Use Cases: Molecular evolution analysis Python Libraries:

• Custom parsers for specific PAML programs
• Biopython: Basic PAML parsing EDA Approach:
• Model parameter extraction
• Likelihood values
• dN/dS ratio distribution
• Branch-specific results
• Convergence assessment

Protein and Structure Data

.embl - EMBL Format

Description: Rich sequence annotation format Typical Data: Sequences with extensive annotations Use Cases: Sequence databases, genome records Python Libraries:

• Biopython: SeqIO.read('file.embl', 'embl') EDA Approach:
• Feature annotation completeness
• Sequence length and type
• Reference information
• Cross-reference validation
• Feature overlap analysis

.genbank / .gb / .gbk - GenBank Format

Description: NCBI's sequence annotation format Typical Data: Annotated sequences with features Use Cases: Sequence databases, annotation transfer Python Libraries:

• Biopython: SeqIO.parse('file.gb', 'genbank') EDA Approach:
• Feature type distribution
• CDS analysis (start codons, stops)
• Translation validation
• Annotation completeness
• Source organism extraction
• Reference and publication info
• Locus tag consistency

.sff - Standard Flowgram Format

Description: 454/Roche sequencing data format Typical Data: Raw pyrosequencing flowgrams Use Cases: Legacy 454 sequencing data Python Libraries:

• Biopython: SeqIO.parse('file.sff', 'sff')
• Platform-specific tools EDA Approach:
• Read count and length
• Flowgram signal quality
• Key sequence detection
• Adapter trimming validation
• Quality score distribution

.hdf5 (Genomics Specific)

Description: HDF5 for genomics (10X, Hi-C, etc.) Typical Data: High-throughput genomics data Use Cases: 10X Genomics, spatial transcriptomics Python Libraries:

• h5py: Low-level access
• scanpy: For 10X data
• cooler: For Hi-C data EDA Approach:
• Dataset structure exploration
• Barcode statistics
• UMI counting
• Feature-barcode matrix analysis
• Spatial coordinates (if applicable)

.cool / .mcool - Cooler Format

Description: HDF5-based Hi-C contact matrices Typical Data: Chromatin interaction matrices Use Cases: 3D genome analysis, Hi-C data Python Libraries:

• cooler: cooler.Cooler('file.cool')
• hicstraw: For .hic format EDA Approach:
• Resolution analysis
• Contact matrix statistics
• Distance decay curves
• Compartment analysis
• TAD boundary detection
• Balance factor validation

.hic - Hi-C Binary Format

Description: Juicer binary Hi-C format Typical Data: Multi-resolution Hi-C matrices Use Cases: Hi-C analysis with Juicer tools Python Libraries:

• hicstraw: hicstraw.HiCFile('file.hic')
• straw: C++ library with Python bindings EDA Approach:
• Available resolutions
• Normalization methods
• Contact statistics
• Chromosomal interactions
• Quality metrics

.bw (ChIP-seq / ATAC-seq specific)

Description: BigWig files for epigenomics Typical Data: Coverage or enrichment signals Use Cases: ChIP-seq, ATAC-seq, DNase-seq Python Libraries:

• pyBigWig: Standard bigWig access EDA Approach:
• Peak enrichment patterns
• Background signal analysis
• Sample correlation
• Signal-to-noise ratio
• Library complexity metrics

.narrowPeak / .broadPeak - ENCODE Peak Formats

Description: BED-based formats for peaks Typical Data: Peak calls with scores and p-values Use Cases: ChIP-seq peak calling output Python Libraries:

• pybedtools: BED-compatible
• Custom parsers for peak-specific fields EDA Approach:
• Peak count and width distribution
• Signal value distribution
• Q-value and p-value analysis
• Peak summit analysis
• Overlap with known features
• Motif enrichment preparation

.wig - Wiggle Format

Description: Dense continuous genomic data Typical Data: Coverage or signal tracks Use Cases: Genome browser visualization Python Libraries:

• pyBigWig: Can convert to bigWig
• Custom parsers for wiggle format EDA Approach:
• Signal statistics
• Coverage metrics
• Format variant (fixedStep vs variableStep)
• Span parameter analysis
• Conversion efficiency to bigWig

.ab1 - Sanger Sequencing Trace

Description: Binary chromatogram format Typical Data: Sanger sequencing traces Use Cases: Capillary sequencing validation Python Libraries:

• Biopython: SeqIO.read('file.ab1', 'abi')
• tracy tools: For quality assessment EDA Approach:
• Base calling quality
• Trace quality scores
• Mixed base detection
• Primer and vector detection
• Read length and quality region
• Heterozygosity detection

.scf - Standard Chromatogram Format

Description: Sanger sequencing chromatogram Typical Data: Base calls and confidence values Use Cases: Sequencing trace analysis Python Libraries:

• Biopython: SCF format support EDA Approach:
• Similar to AB1 format
• Quality score profiles
• Peak height ratios
• Signal-to-noise metrics

.idx - Index Files (Generic)

Description: Index files for various formats Typical Data: Fast random access indices Use Cases: Efficient data access (BAM, VCF, etc.) Python Libraries:

• Format-specific libraries handle indices
• pysam: Auto-handles BAI, CSI indices EDA Approach:
• Index completeness validation
• Binning strategy analysis
• Access performance metrics
• Index size vs data size ratio