A haplotype is a bundle of DNA differences on a single chromosome that you inherited together from one parent.1Wikipedia. Haplotype Because you have two copies of most chromosomes, one from your mother and one from your father, you carry two haplotypes for most stretches of DNA. That is why the same word turns up in ancestry tests, transplant matching, and drug-response reports: it tells you which pieces of DNA travel together.
What a Haplotype Actually Is
Your DNA differs from other people’s at millions of single spots, called variants, plus small insertions and deletions. A haplotype is a set of such variants sitting next to one another at the same location that tend to be inherited together.2Integrated DNA Technologies. Genotyping: Terms to know
Your genotype at a spot lists the variants on both chromosome copies together.3Integrated DNA Technologies. Genotyping: Terms to know A haplotype looks at one chromosome copy at a time rather than the pair.4Wikipedia. Haplotype Two people can therefore have the same genotype yet different haplotypes, depending on which variants share a copy.
At three spots, your pair might read A/G, C/T, and T/T. One copy might read A-C-T and the other G-T-T, and each is a haplotype. Listing only “A/G, C/T, T/T” gives the genotype, and it hides which letters share a copy.
A haplotype has no fixed length. It can span a few variants or a whole gene region, and in mitochondrial DNA the entire small genome acts as one haplotype.
A haplotype is also different from a gene. A gene is defined by its job, such as carrying instructions for a protein, while a haplotype is defined by which variants sit together on one copy. One gene can come in several haplotype versions, and a haplotype can cover part of a gene, a whole gene, or several genes at once.
The word haplogroup is easy to confuse with haplotype. A haplogroup is a family of related haplotypes that share a deep common ancestor. Ancestry companies report haplogroups as letters and numbers for maternal and paternal lines.
How Haplotypes Pass From Parents to Children
Each egg or sperm carries one copy of each of 23 chromosomes, so a child receives one haplotype from the mother and one from the father at each location. The copy a parent passes on is usually not exactly one of the parent’s own two copies. During egg and sperm formation, the parent’s paired chromosomes exchange segments in events called crossovers (recombination).5PubMed Central. Meiotic Recombination: The Essence of Heredity
These crossovers are needed for chromosomes to separate correctly, and they create new mixes of the grandparents’ variants.6PubMed Central. Meiotic Recombination: The Essence of Heredity New variants first appear by mutation, and reshuffling then arranges them into new haplotypes.7Nature Genetics. Reciprocal crossover asymmetry and meiotic drive in a human recombination hot spot Reshuffling changes combinations but does not on its own make any variant more or less common in a population.8Nature Genetics. Reciprocal crossover asymmetry and meiotic drive in a human recombination hot spot
Follow one haplotype through a family. Your mother holds one haplotype she got from her mother and a different one she got from her father. When she makes an egg, a crossover can join the front part of one to the back part of the other, so the egg’s copy is a patchwork. You inherit that patchwork, so you carry a haplotype neither grandparent had whole, though every piece of it came from them.
Variants very close together are rarely split by a crossover, so they stay linked across many generations.9PubMed. Patterns of linkage disequilibrium in the human genome Long shared haplotypes therefore usually mean recent shared ancestry, while short ones reflect ancient ancestry.
Siblings differ because each egg and sperm gets its own crossover pattern. Two siblings can inherit the same or different haplotypes from a parent at any spot, and at any one location they have roughly a one-in-four chance of sharing both parental haplotypes.
You get half your DNA from each parent, but not exactly a quarter from each grandparent. Crossovers make the grandparent share vary from person to person, which is why ancestry percentages can differ between siblings.
Why Haplotypes Shrink Over Generations
Over many generations, repeated reshuffling makes haplotypes shorter.10PubMed Central. Genome-wide scans for footprints of natural selection The farther apart two variants sit, the more likely a crossover falls between them, so distant variants lose their link sooner than close ones.
The genome ends up in blocks where certain variants are found together more often than chance.11PubMed. Patterns of linkage disequilibrium in the human genome The pattern of these associations reflects a population’s history, such as migrations and growth.12PubMed Central. Linkage disequilibrium — understanding the evolutionary past and mapping the medical future
Because variants in a block travel together, testing one can reveal its neighbours. Researchers pick “tag” variants to stand in for whole blocks, which makes gene tests cheaper.
When a helpful variant spreads quickly, the haplotype carrying it rises in frequency along with its neighbours.13PubMed Central. Genome-wide scans for footprints of natural selection A common, unusually long haplotype can therefore point to strong, recent, or ongoing selection, because reshuffling hasn’t had time to break it up, though other population history can also produce one.14PubMed Central. Genome-wide scans for footprints of natural selection
Lactose tolerance is a familiar example. Most mammals stop digesting milk sugar after weaning, but some human populations keep digesting it as adults because of a variant near the lactase gene. In groups with a long history of dairy farming, that variant gave an advantage and spread quickly. It sits on a long shared haplotype, and that long stretch around one variant is the kind of signature researchers look for when searching for recent selection.
Maternal and Paternal Line Haplotypes in Ancestry Tests
Mitochondrial DNA comes only from the mother and doesn’t recombine.15PubMed. Mitochondrial DNA inherited variants are associated with successful aging and longevity in humans It sits in the cell’s energy-making structures, and children get theirs from the mother’s egg, so there is no second copy to swap with. That lets researchers trace maternal lineages and compare them across populations and continents.16PubMed Central. Maternal ancestry and population history from whole mitochondrial genomes
The Y chromosome passes strictly from father to son, and most of it doesn’t recombine.17PubMed. The human Y chromosome: function, evolution and disease It has no matching partner for most of its length, so there is nothing to exchange with during sperm formation. Together with mitochondrial DNA, it keeps a step-by-step record of mutations that can trace ancient migrations, including out of Africa.18PubMed. Use of y chromosome and mitochondrial DNA population structure in tracing human migrations
Without reshuffling, the only changes are rare new mutations, and these accumulate along a line like marks on a family tree. That is why a mitochondrial or Y haplotype can be compared across people to build a branching tree.
On an ancestry report, labels such as H1 for the maternal line or R1b for the paternal line name branches of that tree, and more letters and numbers mean a more specific twig. Two people with the same haplogroup share a common ancestor on that line, but that ancestor may have lived thousands of years ago, so a match doesn’t mean close relatives. Women get only a maternal result since they have no Y chromosome; a woman can learn her paternal line by testing her father or brother.
Each line tracks just one ancestor per generation, so your haplogroup describes a single thread of ancestry, not your whole heritage. Percentage estimates use the reshuffled chromosomes from all your ancestors, so the two can point to different regions without either being wrong.
Haplotypes in Transplants and Donor Matching
HLA genes, the tissue-type markers your immune system uses to spot foreign cells, sit close together and are usually inherited as one haplotype from each parent. Reshuffling rarely splits them, so each parent passes on a whole set. Doctors therefore describe a person’s tissue type as two haplotypes rather than a list of separate genes.
That makes family matching simple to predict. Each full sibling has one of two haplotypes from the mother and one of two from the father, giving four combinations. A sibling shares both haplotypes with you in one of four cases, which is the full match. A sibling shares one in two of four cases, and none in one of four.
Match quality changes outcomes. In bone marrow transplants for leukemia or lymphoma, grafts failed in 2% of patients with a fully matched sibling.19PubMed. Effect of HLA compatibility on engraftment of bone marrow transplants in patients with leukemia or lymphoma Grafts failed in about 12% when the family donor shared only one haplotype.20PubMed. Effect of HLA compatibility on engraftment of bone marrow transplants in patients with leukemia or lymphoma In kidney transplants from siblings, a donor’s unshared HLA from the mother rather than the father gave better 10-year survival, 77% vs 49%.21PubMed. The effect of tolerance to noninherited maternal HLA antigens on the survival of renal transplants from sibling donors
A “half-matched,” or haploidentical, donor shares one of your two HLA haplotypes. Parents and children are always at least half-matched. Such donors are more available but carry the higher graft risk above. Without a family match, hospitals search registries of over 40 million volunteer donors worldwide.22PubMed Central. Bone Marrow Donor Matching: HLA Typing, Sibling Probability & Registry Searches Unrelated registry donors can match by chance even without shared parents.
Family members tested as donors give a cheek swab or blood sample, which is typed for HLA. The results show whether you share zero, one, or two haplotypes with the patient.
Haplotypes and How You Respond to Medicines
A pharmacogenetic report may list a gene such as CYP2D6 with entries like *1/*4; each of these entries is a star allele, a named haplotype of a liver enzyme gene.23PubMed. StellarPGx: A Nextflow Pipeline for Calling Star Alleles in Cytochrome P450 Genes Each number names one of your two copies of the gene, one from each parent. Commonly, *1 is the standard reference version, and other numbers name versions with particular variant patterns, some of which work less well.
Your two star alleles together predict whether you’re a poor, normal, or fast metabolizer, which is what a prescriber uses to tailor drugs and reduce side effects.24PubMed Central. GenoStaR: An R Package for Genotype to Star Allele Conversion for Major Cytochrome P450 Family of Genes Rare uncharacterized variants explain about 1.5 to 17.5% of inherited differences in these genes, so a report can miss some.25PubMed Central. The genetic landscape of major drug metabolizing cytochrome P450 genes—an updated analysis of population-scale sequencing data
Show the report to your doctor or pharmacist before you start a new prescription. Medicines that can be affected include some antidepressants, pain relievers such as codeine, and the blood thinner clopidogrel. Your inherited result does not change with age, so you can keep a copy and reuse it.
Whether two variants sit on the same copy or different copies can change their effect.26PacBio. Sequencing 101: ploidy, haplotypes, and phasing — how to get more from your sequencing data For recessive diseases, two harmful variants on separate copies break both copies, while on one copy the other copy still works.27PacBio. Sequencing 101: ploidy, haplotypes, and phasing — how to get more from your sequencing data
Carrier screening shows why this matters. A couple may each learn they carry a variant for a recessive condition. If one person has two variants, it matters whether they’re on the same copy, and labs may test parents to settle which copy each variant is on.
How Labs Figure Out Your Two Haplotypes
Standard genetic tests read both copies mixed together, so sorting variants by parent is a separate step.28PacBio. Sequencing 101: ploidy, haplotypes, and phasing — how to get more from your sequencing data Computer methods estimate the most likely split by comparing you with haplotypes common in the population.29PubMed Central. Haplotype phasing: Existing methods and new developments These methods are accurate, getting about 3 sorting errors per 1,000 mixed positions.30bioRxiv. A Benchmark of Modern Statistical Phasing Methods Consumer chips estimate haplotypes this way. The estimates are good for common variants but less certain for rare ones, so medical decisions on rare variants usually need confirmatory testing.
Long-read sequencing reads stretches long enough to show directly which variants share a copy.31PubMed Central. A Long-Read Sequencing Approach for Direct Haplotype Phasing in Clinical Settings In a clinical test of this approach, enough reads per region gave the correct haplotype in every sample.32PubMed Central. A Long-Read Sequencing Approach for Direct Haplotype Phasing in Clinical Settings
Testing parents can settle the question too. A child gets one copy from each parent, so if a variant is absent in one parent, it must have come from the other. That lets a lab assign many variants to a parent by simple comparison. Some positions can’t be settled this way, such as when all three people share the same mixed result, and those need statistics.
Trio testing, sequencing the child and both parents together, is common when a child has an unexplained condition, because it shows which variants were inherited and which are new. That is why a lab may ask for parental samples even if the parents have no symptoms.