Genetics / genomics · BEGINNER LESSON
Haplotype
A haplotype is a set of alleles, meaning variant forms of a DNA marker, that are inherited together in a genomic region. In these sources, haplotypes are often defined from biallelic markers such as single nucleotide polymorphisms (SNPs) and can be used to study traits or selection patterns.
What a haplotype is and what is measured
A haplotype is not a single mutation. It is a pattern of nearby genetic variants that tend to travel together through generations because they are linked, especially when linkage disequilibrium (LD) is high. LD means some markers are associated more often than expected if they were inherited independently. Studies measure which variants occur together, how many unique haplotypes exist, and how common each one is in a sample or population. In breeding or association studies, haplotypes can be compared with single markers such as SNPs to see which approach predicts traits better.
[1] Haplotype applications in genomic selection - PMC - NIH [2] Haplotypes and Machine Learning Improve Genomic ... - PMC [3] Protocol for the identification of selected genes and haplotype ...
How haplotypes are identified
One approach is to build haplotype blocks, which are genomic regions showing little to no historical recombination, meaning little evidence that DNA segments were broken up and reshuffled in the past. Some methods use fixed windows, but these assume every region has similar haplotype length. LD-based methods instead use LD thresholds or rules such as the four-gamete rule, which defines a block as having no internal recombination. More advanced sequencing approaches can recover larger, chromosome-scale haplotypes, while block methods usually represent only partial haplotypes.
[1] Haplotype applications in genomic selection - PMC - NIH [3] Protocol for the identification of selected genes and haplotype ...
Why haplotypes are useful
Haplotypes can capture local epistasis, which means interactions among genetic variants in a small region. That can improve trait prediction and help find rare alleles or genomic regions missed by SNP-only analyses. Because a haplotype may combine several variants, it can represent more genetic diversity than a single biallelic marker. In genomic prediction, haplotypes may reduce false positives and false negatives, but they can also lower accuracy if too many similar predictors cause multicollinearity, a strong correlation among predictors that can increase overfitting.
[1] Haplotype applications in genomic selection - PMC - NIH [2] Haplotypes and Machine Learning Improve Genomic ... - PMC
Limitations and sources of bias
Haplotype results depend on accurate genotype calls and phasing, the process of assigning variants to the same copied chromosome. Missing data, poor phasing, batch effects, or inconsistent reference genomes can bias haplotype counts and regional comparisons. Fixed-length blocks may be inaccurate where recombination rate, selection, inbreeding, or mutation make haplotype length vary across the genome. Inbreeding and low LD can reduce gains from haplotype-based prediction, and reference-based methods may miss novel or rare haplotypes.
[1] Haplotype applications in genomic selection - PMC - NIH [3] Protocol for the identification of selected genes and haplotype ...
SEE THE IDEA
How haplotype analysis works
SNPs or other markers in one genomic region.
LD keeps some variants associated across generations.
A region with little historical recombination.
Counts, frequencies, or prediction features for traits or selection.
Nearby variants are grouped by shared inheritance into blocks, then measured for frequency or trait association.
Haplotypes summarize linked variants in a region, but the grouping method and data quality shape the result.
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.