What Is a Chromosomal Deletion and What Causes It?

A chromosomal deletion is the loss of a segment of DNA from a chromosome, ranging from a single gene to a stretch containing hundreds of genes. These deletions can happen during the formation of eggs or sperm, during early embryonic development, or even in the cells of an adult’s body over a lifetime. The consequences span an equally wide range: some deletions cause severe developmental syndromes detectable before birth, others drive cancer, and a surprising number have no obvious effect at all. What makes a deletion harmful depends less on its raw size than on which genes sit in the missing stretch and how sensitive the body is to losing a copy of them.

How DNA Gets Deleted

Chromosomal deletions are not caused by a single process. Several distinct molecular mechanisms can carve out a piece of a chromosome, and understanding which one occurred often matters for predicting whether the same deletion could happen again in a family.

The most studied mechanism is called nonallelic homologous recombination. During the formation of eggs and sperm, chromosomes line up and swap segments with their partner. Normally this swapping is precise, but when two stretches of DNA that look similar but sit at different positions on the chromosome get mistakenly paired, the crossover removes the DNA between them. The result is a deletion on one chromosome and a duplication on the other. This process accounts for many recurrent deletions, meaning the same stretch of DNA is lost in unrelated individuals, because the same misleading repeat sequences keep tripping up the cellular machinery.

1PubMed Central. Frequency of nonallelic homologous recombination is correlated with length of homology: evidence that ectopic synapsis precedes ectopic crossing-over

Not all deletions involve look-alike sequences, though. When a chromosome breaks in two places and the cell’s repair crew stitches the loose ends together without a template to guide them, the segment between the breaks is lost. This repair-by-guesswork, known as non-homologous end joining, can produce deletions at virtually any location in the genome, including spots where there are no repetitive sequences to blame.

2PubMed. Novel genomic insertion-deletion in MLH1: possible mechanistic role for non-homologous end-joining DNA repair

Repetitive elements scattered throughout our genomes also play a role. Short, mobile genetic elements called Alu sequences make up a substantial fraction of human DNA, and recombination between two nearby Alu elements can delete the DNA between them. A comparison of human and chimpanzee genomes identified roughly 492 human-specific deletions, totaling about 400 kilobases of lost DNA, attributable to Alu-mediated recombination alone since the two species diverged.

3PubMed Central. Human genomic deletions mediated by recombination between Alu elements

Another category of deletion arises at so-called common fragile sites, specific regions of the genome that are inherently difficult for the cell to copy. These sites tend to contain long stretches of repetitive DNA that can fold into unusual structures, stalling the copying machinery and leaving the region incompletely replicated. Under stress, the stalled regions can break, and the resulting repairs sometimes remove chunks of DNA. Research has shown that replication stress at one well-known fragile site produced microdeletions closely resembling those found in cancer cells, providing direct evidence that copying problems can generate the same types of deletions seen in tumors.

4PubMed Central. Replication stress induces tumor-like microdeletions in FHIT/FRA3B5PubMed Central. Insights into common fragile site instability: DNA replication challenges at DNA repeat sequences

Environmental Triggers and Parental Age

While most chromosomal deletions arise from errors in the cell’s own copying and repair processes, outside forces can raise the odds. Ionizing radiation, whether from medical imaging, occupational exposure, or environmental sources, is one of the best-documented triggers. Even low doses of gamma radiation have been shown to produce chromosomal aberrations, including deletions, in human blood cells.

6PubMed. Effects of low-dose gamma radiation on DNA damage, chromosomal aberration and expression of repair genes in human blood cells

Certain chemicals can also damage DNA in ways that lead to deletions. Alkylating agents used in chemotherapy, benzene and its metabolites, and some industrial solvents are recognized clastogens, meaning they break chromosomes. The deletions they produce are mostly somatic, occurring in the body’s own cells rather than in eggs or sperm, and they are one reason why some cancer treatments themselves carry a small risk of causing a second, unrelated cancer later.

Parental age is a more nuanced factor. Advanced maternal age is well established as a risk factor for whole-chromosome errors like trisomy 21, but the relationship between parental age and structural rearrangements like deletions is less straightforward. One review concluded that paternal age does not clearly increase the risk of numerical or structural chromosomal anomalies in offspring.

7PubMed. Are children of older fathers at risk for genetic disorders?

However, more recent work has noted that spontaneous mutations accumulate with greater frequency during testicular aging, and there is an increased incidence of some chromosomal aberrations in children conceived by older fathers.

8PubMed. Age-Dependent De Novo Mutations During Spermatogenesis and Their Consequences

The tension between these findings suggests that while the per-cell error rate in sperm likely does climb with age, the increase in structural deletions specifically is modest compared to the well-known effect of maternal age on chromosome-number errors.

Why Losing a Piece of Chromosome Causes Problems

You carry two copies of almost every gene, one inherited from each parent. In many cases, having just one working copy is enough. But certain genes are dosage-sensitive: the body needs both copies producing their normal amount of protein, and losing one copy drops the output below a critical threshold. This phenomenon is known as haploinsufficiency, and it is the primary reason chromosomal deletions cause disease.

9PubMed. Causes and effects of haploinsufficiency

Research into why certain genes are haploinsufficient has revealed an interesting wrinkle. It is not simply that the cell needs more protein than one gene copy can supply. Many haploinsufficient genes are also toxic when overexpressed, meaning the cell is walking a tightrope: too little protein causes disease, but the gene cannot be cranked up to compensate without causing a different problem. These genes occupy a narrow dosage window, which is why a deletion removing just one copy can have outsized consequences.

10PubMed Central. Why haploinsufficiency persists

Gene dosage sensitivity helps explain why two deletions of similar size on different chromosomes can have wildly different outcomes. A deletion that removes a megabase of DNA in a region packed with dosage-sensitive developmental genes will produce a recognizable syndrome. A deletion of the same size in a gene-poor region may go unnoticed for a lifetime.

11PubMed Central. Gene Dosage Sensitivity and Human Genetic Diseases

Beyond Missing Genes: Disrupted Regulatory Architecture

The consequences of a deletion sometimes extend beyond the genes it removes. The genome is organized into neighborhoods called topologically associating domains, where genes and their regulatory switches are kept in close contact within looped-off sections of DNA. A deletion that removes the boundary between two such neighborhoods can let a regulatory switch from one domain reach into the adjacent domain and activate a gene it was never supposed to control. The result is misexpression of a gene that is still physically present and intact.

This principle was demonstrated in mouse models of limb malformations, where deletions of boundary regions allowed regulatory elements normally controlling one gene to instead activate nearby genes, producing complex limb defects that mirrored human conditions.

12Cell. Deletion of Chromatin Boundaries and Enhancer Adoption Cause Complex Limb Malformations

Structural variations including deletions can alter the genome’s three-dimensional architecture by removing these domain boundaries, allowing regulatory elements from neighboring domains to ectopically activate genes and cause disease.

13Trends in Genetics. What Is a Chromosomal Deletion and What Causes It?

This means that the list of genes physically missing from a deleted segment does not tell the whole story. A deletion that looks small and genetically “minor” can still cause disease if it disrupts the spatial organization of the genome in a way that scrambles gene regulation nearby.

Recognizable Syndromes Caused by Deletions

Several well-characterized genetic conditions result directly from specific chromosomal deletions. Two of the most studied examples illustrate how different deletions produce very different clinical pictures.

Cri du Chat syndrome results from a deletion on the short arm of chromosome 5. Affected infants have a distinctive high-pitched cry that gives the syndrome its name (French for “cry of the cat”), along with developmental delays, intellectual disability, and characteristic facial features.

14Academic Press. Non-Invasive Prenatal Screening of Rare Fetal Genetic Diseases Research using animal models has confirmed that the deletion produces pronounced deficits in social behavior, cognition, and anxiety, along with neuronal abnormalities in key brain regions.15PubMed Central. Behavioral Abnormalities, Cognitive Impairments, Synaptic Deficits, and Gene Replacement Therapy in a CRISPR Engineered Rat Model of 5p15.2 Deletion Associated With Cri du Chat Syndrome Beyond the deleted genes themselves, patients show DNA methylation changes in genes on other chromosomes that are linked to their most common symptoms, including developmental delay and microcephaly, suggesting the deletion’s effects ripple outward across the genome.16PubMed Central. Cri du chat syndrome patients have DNA methylation changes in genes linked to symptoms of the disease

DiGeorge syndrome, also called 22q11.2 deletion syndrome, arises from a microdeletion on chromosome 22. It is one of the most common microdeletion syndromes, affecting roughly one in every few thousand live births. Symptoms include congenital heart defects, immune dysfunction, low calcium levels, palatal abnormalities, and an elevated risk of psychiatric conditions. The gene TBX1, which sits within the deleted region, is thought to be a major driver of the heart and craniofacial features.

14Academic Press. Non-Invasive Prenatal Screening of Rare Fetal Genetic Diseases

Other well-known deletion syndromes include Williams syndrome (a deletion on chromosome 7 affecting about 26 genes), Prader-Willi syndrome (a deletion on chromosome 15 inherited from the father), and Angelman syndrome (a deletion in the same region of chromosome 15 but inherited from the mother). The Prader-Willi/Angelman pair is a striking illustration of how the parent of origin matters: the same deletion produces two completely different conditions depending on which parent contributed the affected chromosome, because different genes in the region are silenced depending on parental origin.

Chromosomal Deletions in Cancer

Not all chromosomal deletions are inherited or present from birth. Somatic deletions, those arising in a person’s own cells during their lifetime, play a central role in cancer. In fact, chromosomal deletion has been described as both the earliest and the most frequent type of somatic genetic alteration during the development of tumors.

17PubMed. Chromosomal deletions and tumor suppressor genes in prostate cancer

Deletions drive cancer in several ways. Sometimes a person inherits one faulty copy of a tumor suppressor gene, and a somatic deletion removes the remaining good copy, a process called loss of heterozygosity. This two-hit model is one of the oldest concepts in cancer genetics. Other times, a deletion removes just one copy of a tumor suppressor, and the reduced output from the single remaining copy is insufficient to keep cell growth in check. And in some tumors, both copies of a gene are deleted entirely, shutting it down completely.

17PubMed. Chromosomal deletions and tumor suppressor genes in prostate cancer

Common fragile sites, the regions prone to breakage during DNA replication stress described earlier, are frequently involved in cancer-associated deletions. Experiments have shown that replication stress alone can produce microdeletions at these sites that closely mimic those found in real tumors, supporting the idea that everyday copying errors during cell division contribute to cancer development.

4PubMed Central. Replication stress induces tumor-like microdeletions in FHIT/FRA3B

When Deletions Affect Only Some Cells

A person does not always carry a deletion in every cell. When a deletion arises after fertilization, during the early cell divisions of an embryo or later in life, only the cells descended from the one where the error occurred will carry the deletion. The rest of the body’s cells remain normal. This patchwork state is called mosaicism, and it can make deletions harder to detect and more variable in their effects.

Mosaic deletions are rarer than those present in every cell, but they may be underrecognized. A study that screened 100 families whose children had apparently brand-new (de novo) deletions found that four parents actually carried the same deletion at low levels in their blood cells, even though standard clinical testing had called them normal.

18American Journal of Human Genetics. Parental Somatic Mosaicism Is Underrecognized and Influences Recurrence Risk of Genomic Disorders

This matters for genetic counseling: if a parent carries a deletion in a fraction of their cells, including potentially in their reproductive cells, the chance of having another affected child is higher than the near-zero recurrence risk typically quoted for a de novo event.

Mosaicism can also arise from the body’s own attempts at repair. In one documented case, a mosaic terminal deletion of chromosome 20 was traced back to a somatic repair event, where the cell apparently tried to fix a germline deletion through recombination, succeeding in some cell lineages but not others.

19PubMed. Mosaic deletion of 20pter due to rescue by somatic recombination

Ring Chromosomes and Compound Rearrangements

Sometimes a deletion is part of a more complex rearrangement. Ring chromosomes form when both tips of a chromosome break off and the remaining arms fuse into a circle. The broken tips, which carry genes, are lost. A study of 33 different ring chromosomes found that about a fifth also carried duplications alongside their deletions, producing a compound abnormality more complex than a simple ring.

20Journal of Medical Genetics. Duplications in addition to terminal deletions are present in a proportion of ring chromosomes: clues to the mechanisms of formation

Ring chromosomes form through several distinct routes: breaks in both arms followed by end-to-end fusion, a break in one arm followed by fusion with the other arm’s tip region, or even direct telomere-to-telomere fusion.

21PubMed Central. Mechanisms of ring chromosome formation, ring instability and clinical consequences

Rings are also inherently unstable during cell division. They tend to be lost or broken during mitosis, creating a mosaic situation where some cells carry the ring and others do not. This instability makes clinical outcomes especially hard to predict.

Detecting Deletions Before and After Birth

The tools for finding chromosomal deletions have improved dramatically. Older techniques like standard karyotyping can spot large deletions visible under a microscope, but they miss anything smaller than about five million base pairs. Chromosome microarray analysis, which became widely used in the 2000s, can detect much smaller microdeletions and microduplications throughout the genome, enabling the discovery of many syndromes that were previously invisible to testing.

22Europe PMC. The genetics of microdeletion and microduplication syndromes: an update

For pregnancies, non-invasive prenatal testing that analyzes fragments of fetal DNA circulating in the mother’s blood has extended screening to include common microdeletions. A systematic review found that these tests achieved sensitivity ranging from about 85% to 100% and specificity above 95% for common microdeletion syndromes, depending on the technology used.

23PubMed Central. Validity and Utility of Non-Invasive Prenatal Testing for Copy Number Variations and Microdeletions: A Systematic Review

One validation study reported detection rates of about 98% for 22q11.2 deletion (DiGeorge syndrome) and 100% for several other syndromes including Cri du Chat, with false-positive rates below 1%.

24American Journal of Obstetrics and Gynecology. Noninvasive screening for fetal microdeletion syndromes

These numbers sound reassuring, but there is a catch. Because microdeletion syndromes are individually rare, even a low false-positive rate can mean that a positive screening result is wrong more often than it is right, depending on the condition. Prenatal screening for microdeletions is best understood as a flag that warrants confirmatory testing through amniocentesis or chorionic villus sampling, not as a diagnosis in itself.

Deletions as an Evolutionary Force

It is easy to think of chromosomal deletions as purely destructive, but over evolutionary timescales they have been a surprisingly productive source of adaptation. The avian genome is a good example: birds have notably compact genomes compared to other vertebrates, and that compactness arose largely through lineage-specific erosion of repetitive elements, large segmental deletions, and gene loss over millions of years.

25PubMed. Comparative genomics reveals insights into avian genome evolution and adaptation

In mammals, a systematic survey found multiple gene losses that likely contributed to adaptations in aquatic and flying species, suggesting that deletion-driven gene loss may be a more widespread mechanism for evolutionary adaptation than previously thought.

26Nature Communications. A genomics approach reveals insights into the importance of gene losses for mammalian adaptations

Laboratory experiments in yeast have demonstrated the mechanism directly: after individual genes were deleted, the organisms often acquired compensatory mutations that restored or even exceeded their original fitness. Cells that lost highly connected genes in particular showed an increased capacity to evolve diverse new traits, essentially opening up alternative evolutionary paths that were not available to the intact organism.

27PubMed Central. Gene Loss Predictably Drives Evolutionary Adaptation

Emerging Therapeutic Strategies

For most of their history, chromosomal deletion syndromes have been treated symptom by symptom, because the underlying genetic loss could not be reversed. That picture is beginning to shift with advances in gene therapy and gene editing. In Angelman syndrome, which typically results from loss of the maternal copy of the UBE3A gene, researchers are developing antisense oligonucleotides and gene therapies designed to reactivate the intact but silenced paternal copy of the gene, effectively working around the deletion rather than replacing it.

28PubMed Central. Prenatal treatment path for angelman syndrome and other neurodevelopmental disorders

In animal models, researchers have used CRISPR-based gene editing to restore function lost to Y-chromosome deletions that cause infertility, with treated mice resuming sperm production and producing offspring. These results remain far from clinical use in humans, but they demonstrate that the consequences of a chromosomal deletion are not necessarily permanent in principle. Whether gene therapies can be delivered safely and effectively enough to treat deletion syndromes with effects across many tissues and organ systems remains one of the major open questions in clinical genetics.