Most people's mental model of a genetic mutation is a single letter swap — one base pair changed out of billions. That category, called a single nucleotide variant (SNV), is real and important, but it's far from the only kind of mutation that matters. Structural variants are a completely different category: large-scale changes to the structure of a chromosome itself — segments that are deleted, duplicated, inverted, or moved to a different location entirely. Copy number variants (CNVs) are a specific and common type of structural variant, where a person has an abnormal number of copies of a particular DNA segment — one copy instead of the usual two, or three or more instead of two.

Why They're So Easy to Miss

Standard SNP-chip genotyping arrays (the technology behind most budget consumer ancestry-and-traits tests) are specifically designed to check known single-letter positions — they're built to answer "what's at this exact spot," not "is this whole chunk of chromosome duplicated or missing." Whole exome sequencing, despite being far more thorough than a SNP chip, has its own blind spot here too: because most structural variants and CNVs physically extend beyond the narrow, targeted regions exome capture kits are designed to sequence, exome data is considered unreliable for detecting them, almost by definition of how the test is built. Whole genome sequencing, because it reads the entire genome including the boundaries between regions, is the most reliable of the common approaches for catching structural variants and CNVs as a matter of course.

3 Mb+
Some clinically significant microdeletions and microduplications span up to several million base pairs — utterly invisible to a test that only checks predetermined single-letter positions.
— Clinical genomics literature on CNV detection

Why These Matter Clinically

Structural variants and CNVs are implicated in a wide range of conditions, from certain congenital developmental disorders and specific microdeletion/microduplication syndromes, to elevated risk contributions in some neuropsychiatric and neurodevelopmental conditions, to cancer, where large-scale chromosomal rearrangements are a hallmark of many tumor types. They're also relevant in reproductive genetics: a parent carrying a "balanced" structural rearrangement (where the total genetic material is normal but rearranged) can be entirely healthy themselves while still facing meaningfully elevated risk of miscarriage or an unbalanced chromosomal condition in offspring — precisely the kind of finding that PGT-SR embryo testing (covered in our advanced family planning guide) is specifically designed to catch.

Key Takeaway

"I got tested and nothing showed up" often means "the test I took wasn't designed to look for this category of mutation" — not "this category of mutation doesn't apply to me." Structural variants and CNVs require a sequencing approach specifically capable of detecting large-scale, whole-region changes, not just spot-checks at predetermined positions.

What To Do With This

If you have a family history of a condition linked to a known structural variant or microdeletion/microduplication syndrome, or unexplained developmental or fertility concerns, make sure whoever orders your testing knows structural variant detection is specifically part of what you need — a standard SNP array or a narrow gene panel may not be built to catch it, even if it comes back looking clean.

Whole Genome Data Catches What Spot-Checks Miss

Dante Labs' whole genome sequencing reads your complete genome, making it far more capable of detecting structural variants and CNVs than SNP-chip or exome-based tests. Use code GENOME for 10% off.

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For the mechanics of embryo-stage structural variant testing, see our advanced family planning guide, and for how WGS compares to narrower testing more broadly, see our whole genome vs. whole exome sequencing guide.