A genetic report says that a segment of DNA is deleted or duplicated. The size may look alarming: thousands or millions of DNA letters. But size alone cannot tell you whether the finding explains a person’s symptoms or what it means for their future.
A copy-number variant, or CNV, changes how many copies of a DNA segment are present. The important questions include which material is affected, whether it is lost or gained, how confidently the change was measured, and what evidence links that change to disease.
First, translate the report into a description
A deletion removes material; a duplication adds a copy of a segment. Some structural changes affect health, while others do not. MedlinePlus explains structural chromosome changes and why location and disruption of gene function matter alongside size.
| Report detail | Why it matters |
|---|---|
| Chromosome, coordinates and reference assembly | They identify the reported interval. Coordinates need a genome version, such as GRCh37 or GRCh38. |
| Loss or gain, and copy-number state | A deletion and a duplication of the same region need different interpretation. |
| Genes or regions involved | They connect the measurement to possible biological effects. |
| Method and boundary uncertainty | The assay may estimate an interval without resolving its exact structure. |
| Classification and clinical explanation | The evidence about the CNV and its relevance to this person are related but separate conclusions. |
For a typical non-mosaic autosomal region, two copies are expected. A report of one copy can indicate a deletion, while three can indicate a duplication. Sex chromosomes, mosaic findings and more complex changes need their own context. Ask the laboratory to explain its notation rather than applying this simple example to every report.
Example 1: the smaller finding may be more informative
Fictional teaching example. One person has a 200-kilobase deletion that removes an established dosage-sensitive gene. Another has a 2-megabase duplication in a different region. A relative assumes that the larger finding must be ten times more serious.
Those invented sizes do not provide a severity scale. The effects depend on the material involved and the evidence for the particular loss or gain. Neither finding can be classified from these descriptions alone.
A better question is: “Which part of this interval is thought to matter, and what supports that interpretation?” This invites an explanation of the evidence rather than a comparison of two large numbers.
Why losing and gaining a copy are different
ClinGen’s dosage-sensitivity resource evaluates evidence for haploinsufficiency and triplosensitivity separately. Haploinsufficiency means that loss of one functional copy can cause disease. Triplosensitivity concerns disease associated with an extra copy. Evidence for one direction does not automatically establish the other.
A useful real example is PMP22. An extra copy is associated with Charcot–Marie–Tooth disease type 1A, while deletion of one copy is associated with hereditary neuropathy with liability to pressure palsies. The MedlinePlus PMP22 guide explains these different effects. The lesson is to preserve the direction of the change, not simply report “a PMP22 finding.”
When checking a dosage resource, record the exact gene or region, evidence assessment and date. A missing or incomplete curation is not proof that a finding is harmless.
Example 2: “duplicated” may leave a structural question
Fictional teaching example. A report identifies an extra copy of several exons within a gene. The family is told there is “more of the gene,” and assumes that the extra material must produce extra normal protein.
The result may not establish that. The arrangement of the added material and its effects on the transcript can matter. A partial duplication is not necessarily equivalent to an intact extra gene. The laboratory may need more information to characterize the event or explain its predicted consequence.
GeneReviews’ testing overview discusses what chromosomal microarrays and other methods can and cannot resolve. A copy-number measurement does not necessarily show the physical location or orientation of the extra segment. Ask whether the reported interpretation depends on structural information that the assay did not establish.
Example 3: finding it in a parent changes the evidence, not automatically the answer
Fictional teaching example. A child with developmental concerns and a parent who reports no difficulties share the same CNV. The family asks whether this proves that the CNV is benign.
Inheritance is relevant, but the conclusion depends on the specific condition, the parent’s assessment and the expected variation in expression. Some disease-associated changes show reduced penetrance or variable expressivity. Conversely, a new finding absent from parental samples is not automatically pathogenic.
MedlinePlus explains penetrance and expressivity. Useful family information includes ages, examinations and relevant features; “unaffected” can otherwise mean anything from a detailed assessment to the absence of a remembered diagnosis.
Keep classification separate from the individual explanation
The ACMG/ClinGen constitutional CNV standards use an evidence-based framework and distinguish variant classification from its significance for an individual patient’s diagnosis. A classified CNV may explain some findings without explaining every symptom. A VUS remains uncertain even when it is an appealing candidate.
The report discussion should therefore answer two questions: “What does the evidence establish about this genomic change?” and “How well does it explain this person’s presentation?” Read our VUS guide if the laboratory cannot yet reach a clear classification.
A useful set of questions for the appointment
- What exactly was found? Keep the complete interval, genome version and copy-number state.
- Which mechanism matters? Ask whether the interpretation concerns dosage, gene disruption or another effect.
- How certain is the measurement? Would confirmation or structural clarification answer a specific question?
- What would family testing add? Ask how different results could change the assessment.
- What follows now? Separate established clinical needs from investigations intended to resolve uncertainty.
The variant research worksheet can help organize evidence questions, and the care directory links to professional resources. Keep a copy of the report and the agreed plan so that later consultations start from the same description of the finding.