What Happens When a Piece of DNA Is Missing?

A missing piece of DNA can do nothing at all or it can reshape an entire body, and the difference depends almost entirely on what was deleted, how much was removed, and which copy of the genome took the hit. Deletions range from a single lost letter of genetic code up to millions of base pairs spanning dozens of genes. Some fall in stretches of the genome that tolerate the loss without any detectable consequence, while others knock out a gene so critical that a single missing copy causes a recognizable medical syndrome. The biology of “missing DNA” is really a story about context.

How DNA Deletions Happen

DNA does not simply fall apart at random. Most deletions arise during the cell’s attempts to copy or repair itself, and a few specific error-prone processes account for the majority of them. Small deletions of a few base pairs tend to occur through a copying glitch called slippage, where the DNA replication machinery encounters short repeated sequences and accidentally skips over one copy, deleting the stretch in between. An analysis of disease-causing microdeletions and microinsertions found that slippage involving short tandem repeats, combined with secondary structures in the local DNA, can explain most of these small losses.¹

Larger deletions often arise when the cell’s repair machinery mishandles a double-strand break, where both strands of the DNA helix snap. One common repair pathway, called nonhomologous end-joining, essentially glues broken ends back together without a template to guide the repair. If two breaks happen near each other, the repair system can join the wrong ends, deleting everything between them. In yeast studies, this process produced deletions with a median size of roughly 100 base pairs.² A related mechanism uses tiny stretches of matching sequence at the break sites to guide the joining. In a study of deletions affecting a gene called FOXL2, short matching sequences of one to 66 base pairs were found at over 90% of the breakpoint junctions, confirming that these microhomology-mediated processes drive a large share of disease-relevant deletions.³

When Missing DNA Does Not Matter

Not every deletion is a disaster. A surprisingly large fraction of the human genome appears to tolerate copy-number losses without causing detectable harm. One mapping effort estimated that roughly 37 to 41% of the genome falls in regions classified as benign for copy-number variation, depending on the database used.⁴ These stretches often sit between genes or in regions whose functions are redundant. People walk around carrying inherited deletions in these zones without ever knowing. Population-scale genome sequencing projects have catalogued thousands of such “normal” deletions shared among healthy individuals.

Cells also have built-in buffering systems that can compensate when a gene loses one of its two copies. Protein-level dosage compensation, where the cell adjusts how much protein it makes or degrades, can mask the effect of a lost gene copy. In human cells, roughly a quarter of genes and well over half of genes encoding parts of multi-protein complexes show evidence of this kind of compensation.⁵ The cell’s quality-control machinery, including specific enzymes that tag and destroy excess protein subunits, actively maintains the right balance.⁶

When One Working Copy Is Not Enough

For some genes, however, a single remaining copy cannot produce enough protein to keep things running normally. This situation, called haploinsufficiency, affects a specific subset of genes that are already limiting for important cellular processes even at their normal two-copy level.⁷ Losing one copy of a haploinsufficient gene is like removing one engine from a two-engine airplane that needs both to stay airborne. The remaining engine works fine on its own terms, but the total output falls below the threshold the system needs.

Heart muscle provides a vivid example. Truncating variants in the gene TTN, which encodes titin, one of the largest proteins in the body and a critical structural component of heart muscle fibers, can cause a form of dilated cardiomyopathy. Recent work showed that using CRISPR-based gene activation to boost output from the remaining functional copy of TTN rescued the heart-muscle defects, confirming that the problem was genuinely one of insufficient dosage rather than a toxic effect from the damaged copy.⁸

Well-Known Deletion Syndromes

Some of the best-studied genetic conditions in medicine result from the loss of a defined chunk of chromosomal DNA. The 22q11.2 deletion syndrome is the most common microdeletion syndrome, affecting roughly one in every 2,000 to 4,000 live births.⁹ The deletion removes about 40 to 50 genes from a specific region of chromosome 22.¹⁰ The resulting effects span multiple organ systems: congenital heart defects, palatal abnormalities, immune deficiency, low blood calcium from underactive parathyroid glands, and kidney anomalies are among the most common features.⁹ Originally described in the 1960s as DiGeorge syndrome based on the triad of immune problems, low calcium, and heart defects, the condition is now understood to extend far beyond that original description, encompassing cognitive delays, behavioral differences, and psychiatric illness.¹¹

What makes 22q11.2 deletion syndrome especially instructive is its variability. Two people carrying the exact same deletion can present very differently, with one having severe heart defects requiring surgery and another having mild speech issues and nothing else. That variability is a recurring theme across deletion syndromes and illustrates why “a piece of DNA is missing” never tells the whole story. The genetic background of the remaining genome, environmental factors during development, and the particular genes affected all shape the outcome.

Same Region, Opposite Syndromes

One of the more striking demonstrations of what a deletion can do involves a stretch of chromosome 15. A roughly four-million-base-pair deletion in the 15q11-q13 region causes either Prader-Willi syndrome or Angelman syndrome, but which one you get depends entirely on which parent contributed the deleted chromosome.¹² Prader-Willi syndrome, caused by loss of the father’s copy, produces severe feeding difficulties in infancy that later reverse into insatiable hunger and obesity, along with intellectual disability and behavioral challenges. Angelman syndrome, caused by loss of the mother’s copy, features a completely different picture: severe intellectual disability, seizures, minimal speech, and frequent laughter.¹³

This happens because of a phenomenon called genomic imprinting, where certain genes are silenced on one parental copy and active only on the other. In the 15q11-q13 region, some genes are expressed only from the father’s chromosome and others only from the mother’s. When the father’s copy is deleted, the paternally expressed genes are gone and the maternally silenced versions cannot compensate, producing Prader-Willi syndrome. When the mother’s copy is deleted, the maternally expressed genes are gone instead, producing Angelman syndrome.¹⁴ The DNA that is missing is nearly the same; the consequences are opposite.

When the Missing DNA Was Not a Gene

Not all consequential deletions remove genes. A growing body of work shows that deletions in the non-coding genome, the vast majority of DNA that does not directly encode proteins, can be just as damaging. Regulatory elements like enhancers act as remote control switches that turn genes on in specific tissues at specific times. Deleting an enhancer can silence a perfectly intact gene. Research has shown that mutations in a pancreas-specific enhancer for the transcription factor PTF1A cause isolated pancreatic agenesis, meaning the pancreas simply fails to form, even though the gene itself is untouched.¹⁵

Another category of non-coding deletion involves the boundaries that organize how DNA folds in three dimensions inside the cell nucleus. The genome is partitioned into neighborhoods called topologically associating domains, and the boundaries between these neighborhoods keep genes and their regulatory elements properly paired. Deleting a boundary can merge two adjacent neighborhoods, allowing enhancers to activate genes they were never supposed to reach. In mouse models, this kind of boundary deletion produced limb malformations by creating novel gene-enhancer interactions that do not exist in normal development.¹⁶ A broader analysis found that seven out of eight boundary deletions tested led to measurable changes in local 3D genome structure, including the merging of adjacent domains.¹⁷

Deletions That Only Hit Some Cells

Everything discussed so far assumes the deletion is present in every cell of the body, as happens when a deletion is inherited or occurs at conception. But deletions can also arise after an embryo has started developing, producing a situation called somatic mosaicism, where some cells carry the deletion and others do not. The consequences depend on when the deletion happened during development and which tissues ended up with the affected cells.¹⁸

A deletion occurring in one cell of a four-cell embryo might affect roughly a quarter of the body. The same deletion occurring in a skin cell of an adult might produce nothing more than an invisible patch of genetically distinct tissue. Cancer is the most prominent consequence of somatic deletion, but mosaic deletions can also produce milder versions of genetic syndromes that would be severe if present in every cell. This makes genetic diagnosis trickier, because a standard blood test might miss a deletion that is present primarily in brain or skin tissue.

Deletions That Turned Out to Be Useful

Missing DNA is not always a liability. Some deletions have been preserved or even favored by natural selection because they confer a survival advantage. The most famous example involves a 32-base-pair deletion in the CCR5 gene, which encodes a receptor on the surface of immune cells. People who carry two copies of this deletion produce no functional CCR5 receptor and are nearly completely resistant to HIV infection, because the virus uses that receptor to enter cells.¹⁹ The deletion is young in evolutionary terms but has reached relatively high frequencies in European populations, suggesting it was driven to prominence by intense positive selection, possibly by resistance to plague or another historic pathogen.²⁰

An even older example is humanity’s inability to make vitamin C. Humans, along with other primates and a few other mammalian lineages, lost the gene for the final enzyme in the vitamin C synthesis pathway. The gene, called GULO, accumulated mutations including deletions that rendered it nonfunctional.²¹ This is why you need to eat fruits and vegetables to avoid scurvy while your dog does not. The loss was tolerable because our fruit-eating ancestors got plenty of vitamin C from their diet, and the body compensated by evolving greater reliance on a transporter gene called SVCT1 to absorb dietary vitamin C. In lineages that lost GULO, SVCT1 became essential, and vice versa, showing how the genome reshuffles its dependencies after a deletion.²²

Deletions and Cancer

Cancer genomes are riddled with deletions. As tumors grow and their cells divide chaotically, chunks of DNA are frequently lost. One pattern that researchers have focused on is loss of heterozygosity, where a cell that originally carried two different versions of a gene loses one version entirely. In normal tissue, having two different alleles of a gene provides a safety net. When a tumor cell loses one allele of an essential gene through deletion, it becomes wholly dependent on the protein produced by the single remaining allele. Researchers have proposed exploiting this vulnerability: by designing drugs or genetic tools that specifically inactivate the retained allele in the tumor, you could selectively kill cancer cells while leaving normal cells, which still have both alleles, unharmed.²³ This is still a research-stage strategy, but it illustrates how deletions in cancer cells create weaknesses as well as strengths.

Detecting Deletions

Finding a deletion depends on its size. Traditional chromosome analysis under a microscope can spot large rearrangements but misses anything smaller than a few million base pairs. Chromosomal microarray analysis pushed detection down to much smaller deletions, and newer methods continue to sharpen the resolution. Optical genome mapping, a technique that images long individual DNA molecules, has shown strong performance in catching known microdeletion syndromes including 22q11.2 deletion (DiGeorge syndrome), Williams-Beuren syndrome, and others, correctly identifying all tested microdeletions and microduplications ranging from 34 kilobases to over four megabases.²⁴ In prenatal testing, optical genome mapping achieved 100% concordance with established methods across over a hundred chromosomal abnormalities including deletions and duplications.²⁵

For pregnancies, noninvasive prenatal testing using fragments of fetal DNA circulating in the mother’s blood can now screen for certain microdeletions. This approach has been used to detect chromosome 15 deletions associated with Prader-Willi and Angelman syndromes, flagging the abnormality before birth and allowing confirmation with more detailed methods.²⁶ Optical genome mapping has similarly proven capable of identifying these 15q11.2-q13.1 deletions and duplications, with molecules spanning the breakpoints providing structural detail that older methods could not offer.²⁷

Therapeutic Strategies for Deletion Disorders

Treating a genetic condition caused by missing DNA poses an obvious challenge: you cannot simply paste the DNA back in the way you might correct a single misspelled letter. But several strategies work around this limitation in creative ways.

For Duchenne muscular dystrophy, where deletions in the dystrophin gene shift the reading frame so no functional protein can be made, a therapy called exon skipping uses short synthetic molecules to trick the cell’s splicing machinery into skipping over the disrupted region. The result is a shorter-than-normal but partially functional dystrophin protein, converting a severe disease into a milder form.²⁸ Several exon-skipping drugs have received regulatory approval for specific dystrophin deletions.²⁹

For haploinsufficient conditions, where the problem is not a toxic product but simply too little of a normal one, a newer approach uses CRISPR-based gene activation to crank up output from the remaining good copy. In mouse models of obesity caused by haploinsufficiency of the Sim1 or Mc4r genes, injecting a CRISPR activation system into the brain reversed the obesity phenotype by boosting expression from the single working allele.³⁰ Gene therapy using adeno-associated virus vectors, which can deliver a working copy of a gene to affected tissues, represents another option and has so far shown a favorable safety profile across multiple therapeutic applications.³¹

Deliberate Deletions in Research

Researchers also create deletions on purpose to study what genes do. Knocking out a gene, by deleting it or disabling it using tools like CRISPR-Cas9, and then watching what happens to the cell or organism is one of the foundational methods in genetics. Modern approaches have streamlined the process considerably. A simplified CRISPR knockout method demonstrated that homologous recombination with short 30-base-pair overlapping sequences could reliably eliminate target genes in yeast, with the resulting knockouts remaining stable under normal growth conditions.³² Systematic deletion of every gene in an organism’s genome, one at a time, has been completed for yeast and is underway or partially completed for other model organisms. These knockout libraries reveal which genes are essential, which are dispensable, and which matter only under certain conditions, building a functional map of the genome that illuminates what each piece of DNA actually does when it goes missing.