Read-Backed Phasing

The core mechanism relies on paired-end reads or long reads that span two or more heterozygous loci. Because a single read originates from one parental chromosome, any variants observed on that read must belong to the same haplotype. This provides direct experimental evidence for phasing, eliminating the need for population-based statistical inference. The minimum requirement is a read that overlaps at least two heterozygous positions.

Effective read-backed phasing depends on specific sequencing library characteristics:

• Insert size: Paired-end libraries with larger insert sizes (e.g., 500bp+) increase the probability of spanning multiple heterozygous variants.
• Long-read technologies: Pacific Biosciences (PacBio) HiFi and Oxford Nanopore reads, which routinely exceed 10-20kb, can phase entire genes or large genomic regions in a single read.
• Heterozygosity density: Phasing resolution is directly proportional to the local density of heterozygous variants in the individual's genome.

Tools like WhatsHap and GATK's ReadBackedPhasing implement this approach by:

• Parsing the CIGAR string and MD tag from BAM files to identify variant positions within each read.
• Constructing a graph where nodes are alleles and edges represent read-backed connections.
• Performing weighted minimum-cut or greedy phasing to resolve the two haplotypes.
• Outputting phased VCF records with haplotype identifiers (e.g., 1|0 or 0|1 genotype fields).

Read-backed phasing offers distinct benefits for clinical and rare variant analysis:

• Phase rare variants: Statistical methods like SHAPEIT rely on linkage disequilibrium from reference panels, which fails for private or de novo mutations. Read-backed phasing directly phases any variant covered by a spanning read.
• No reference panel bias: Results are independent of population allele frequencies, making it essential for underrepresented populations.
• Resolves compound heterozygosity: Critically distinguishes whether two deleterious variants in the same gene are in cis (same copy) or trans (different copies), which determines recessive disease risk.

The primary constraint is phase block length, which is limited by the maximum insert size or read length. Regions with low heterozygosity or gaps between spanned variants remain unphased. Key failure modes include:

• Switch errors: Incorrectly flipping the phase assignment within a block, often caused by somatic mutations or mapping errors.
• Hanging reads: Reads that overlap only one heterozygous site provide no phasing information and are discarded.
• Strand bias: Systematic errors on one DNA strand can create false connections between variants.

Modern production pipelines combine read-backed and statistical phasing for maximum contiguity:

1. Read-backed phasing first establishes high-confidence physical phase blocks from spanning reads.
2. Statistical phasing (e.g., Eagle2, SHAPEIT4) then uses a reference panel to connect these blocks across longer distances where no single read spans the gap.
3. This hybrid approach is standard in large-scale initiatives like the 1000 Genomes Project and All of Us Research Program.