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The copy number of a gene is how many copies of that gene a genome carries. People most often meet the term through copy number variation (CNV), where the number of copies of a DNA segment differs between individuals. A person can carry one, three or more copies of a particular gene, and less commonly both copies may be missing. The phrase “gene copy number” and “copy number of gene” mean the same thing; what changes between papers is which of four different things is being counted.
Definition and scope
The National Human Genome Research Institute describes copy number variation as a circumstance in which the number of copies of a specific segment of DNA varies among different individuals’ genomes. The varying segments may be short or include thousands of bases, and an affected region may or may not contain a gene. MedlinePlus Genetics adds that CNV results from insertions, deletions and duplications of large segments of DNA that are at least one thousand nucleotides long.
So “the copy number of a gene” is a count attached to a stretch of genomic DNA, not a property of the gene’s sequence.
How common is copy number variation?
MedlinePlus Genetics states that more than 10 percent of the human genome appears to contain differences in gene copy number. It also states that much of this variation does not affect health or development. Changes in copy number can influence the activity of genes and the functioning of the proteins made from them, and some changes affect a person’s risk of disease.
NHGRI gives a concrete example: the amylase gene. Populations with traditionally starch-heavy diets tend to carry more copies of it, which illustrates that small changes in the number of copies of a section of a genome can have large consequences.
Four things people mean by “copy number”
Searches for this term mix several different quantities. Separating them is the quickest way to avoid reading the wrong method or the wrong literature.
| What is counted | Scope | Where to read more |
|---|---|---|
| Copies of a gene or DNA segment in a genome (CNV) | Segments from short stretches up to thousands of bases; a segment may or may not contain a gene (NHGRI) | This page |
| Whole chromosomes gained or lost | A gain or loss in the number of chromosomes from the normal 46 is called aneuploidy (MedlinePlus Genetics) | A different concept from gene copy number |
| Plasmid copies per cell | A property of the plasmid and its origin of replication, not of the host genome | Origin of replication in plasmids |
| Relative fold change in a qPCR experiment | A ratio between conditions, not a count of copies | Delta-delta Ct analysis |
How copy number is measured in practice
Assays that report copy number generally fall into two families: those that report a ratio against a reference and those that report an absolute count. The methods themselves are covered on existing CASRAI pages rather than repeated here:
- For absolute counts, see the comparison of digital PCR and qPCR for absolute quantification, which includes a section on copy number variation and gene copy counting.
- For calibrating a real-time PCR measurement against known quantities, see building and validating a qPCR standard curve.
- For how probe-based real-time assays work and what to record, see TaqMan qPCR.
- For sequence-level detection of gains and losses, see DNA sequencing methods.
Where it matters for labs
For a lab, the practical consequence of the definitions above is about reporting. A result stated as “copy number” should say which of the four quantities in the table it is, and whether it is an absolute count or a relative value. A relative value from a delta-delta Ct analysis cannot be quoted as a number of copies, and a plasmid copy number says nothing about the host genome.
The size of the segment matters too. MedlinePlus Genetics defines CNV at one thousand nucleotides and above, so very small gains or losses are outside that definition.
Common misunderstandings
Copy number is not the same as expression level
Copy number changes can influence gene activity (MedlinePlus Genetics), but a count of DNA copies is a statement about the genome, not a measurement of how much gene product is made.
A copy number difference is not automatically a disease variant
MedlinePlus Genetics notes that much copy number variation does not affect health or development. A difference from a reference count is a finding to be interpreted, not a diagnosis.
Aneuploidy is not gene copy number
Aneuploidy is a gain or loss in the number of chromosomes from the normal 46. CNV concerns segments of DNA, which may be as small as a short stretch of bases.
FAQ
What is the copy number of a gene?
It is the number of copies of that gene present in a genome. Individuals can differ: a person can have one, three or more copies of a particular gene, and less commonly both copies may be missing (MedlinePlus Genetics).
What is the difference between gene copy number and copy number variation?
Gene copy number is the count for one gene. Copy number variation is the broader circumstance in which the number of copies of a DNA segment varies among individuals’ genomes, whether or not the segment contains a gene (NHGRI).
How large is a copy number variant?
NHGRI says variants may be short or include thousands of bases. MedlinePlus Genetics describes CNV as involving segments of at least one thousand nucleotides.
Sources
- National Human Genome Research Institute, Talking Glossary of Genomic and Genetic Terms, “Copy Number Variation”: genome.gov/genetics-glossary/Copy-Number-Variation, checked 2026-10-29.
- MedlinePlus Genetics, “What are copy number variations?”: medlineplus.gov/genetics/understanding/mutationsanddisorders/copynumbervariation/, checked 2026-10-29.
- MedlinePlus Genetics, “Chromosomal conditions”: medlineplus.gov/genetics/understanding/mutationsanddisorders/chromosomalconditions/, checked 2026-10-29.
