When a child is offered exome or genome sequencing, the team may also request samples from both biological parents. This is usually called trio sequencing. The parents’ samples help the laboratory understand how the child’s genetic findings arose and whether their inheritance fits a possible diagnosis.
Three samples do not guarantee an answer. Their value is that they can turn an ambiguous finding into a more specific question: is this change new in the child, inherited from a parent, or paired with another change on the other copy of a gene?
What does a trio add?
The person being investigated is called the proband. In a typical trio, the laboratory analyzes the proband and both biological parents. Family-based analysis can narrow the candidates that require detailed review. It does not automatically expand the types of genetic change that the chosen assay can measure.
The ACMG’s 2021 guideline supports considering exome or genome sequencing as a first- or second-tier test for children with specified congenital anomalies, developmental delay or intellectual disability. That recommendation concerns a defined clinical population; it is not a promise that every trio will produce a diagnosis.
| Observation | What it can help assess | What still needs checking |
|---|---|---|
| A variant is detected in the child but not in parental samples. | A possible de novo origin. | Technical quality, family relationships, disease mechanism and phenotype. |
| Two variants come from different parents. | Whether they affect opposite copies of an autosomal gene. | The significance of each variant and whether the condition fits. |
| A variant is also found in a parent. | The family’s inheritance pattern. | The parent’s assessment, penetrance and the expected disease model. |
Example 1: new in the child does not automatically mean causative
Fictional teaching example. A child with developmental difficulties has a rare sequence change that is not detected in either parent’s blood sample. The family asks whether that is the diagnosis.
The finding deserves attention, but absence in the parents is one piece of evidence. The laboratory still needs to assess whether the gene is convincingly associated with the condition, whether this kind of change fits the disease mechanism, and whether the child’s features are compatible. Rare, newly arisen variants can also be unrelated to the presenting problem.
The Deciphering Developmental Disorders study demonstrated the importance of damaging de novo variants using evidence across thousands of families. That population-level evidence does not make every individual de novo variant diagnostic. Our gene–disease validity guide explains why the gene association itself must be checked.
Example 2: two variants need an arrangement, not just a count
Fictional teaching example. A child has two candidate variants in an autosomal gene associated with a recessive condition. The mother carries variant A; the father carries variant B. With reliable genotype and family data, the child’s variants are on opposite gene copies, described as in trans.
If both changes instead sit on the same copy, they are in cis. They would not, by themselves, establish a change affecting each copy. This is why the arrangement, or phase, can matter as much as the number of findings.
Being in trans is informative, but it does not make two uncertain variants pathogenic. Each still needs interpretation.
The ACMG/AMP sequence-variant guidelines place inheritance evidence within a wider classification framework. Our practical ACMG/AMP guide shows how to avoid letting one observation decide the whole case.
Example 3: an apparently unaffected parent needs context
Fictional teaching example. A candidate dominant variant is found in a child and in a parent who reports no symptoms. It is tempting to dismiss the variant immediately.
Ask how the parent was assessed and whether the condition can have age-dependent onset, reduced penetrance or variable expression. For some disease models, a genuinely unaffected older carrier can weigh against a proposed explanation; for others, the observation may be less decisive. Neither “inherited” nor “de novo” replaces the clinical assessment.
MedlinePlus explains reduced penetrance and variable expressivity. Read our guide to penetrance and expressivity alongside the original laboratory interpretation. Useful notes describe the parent’s relevant examination and age, rather than simply recording “normal parent.”
What comparative research actually tells us
In a 2019 head-to-head study by Tan and colleagues, teams analyzed the same 30 cases using singleton and trio approaches. Singleton analysis diagnosed 11 cases; trio analysis diagnosed 12. Parental data also reduced the number of variants needing curation.
This small study illustrates both diagnostic and workflow benefits. It is not a universal estimate of how much trio testing improves yield. The starting population, previous testing and quality of the singleton workflow all affect the comparison. When reading another study, check whether it compares the same cases or groups with different referral histories.
Does a de novo result mean no recurrence risk?
No. A parental blood sample may not reveal low-level mosaicism or a variant confined to reproductive cells. Absence from the tested samples is not proof that recurrence is impossible.
Bernkopf and colleagues’ 2023 study investigated individualized recurrence assessment after a child had a pathogenic de novo variant. It illustrates why origin and mosaicism can matter. It does not justify assigning every family the same probability or requesting specialized samples without a clinical plan. Ask the genetics team how the confirmed diagnosis affects this family’s counselling.
Before samples are collected
- Clarify each person’s role. Are parental samples used for comparison only, or is additional analysis offered?
- Discuss consent separately. Ask about secondary findings and unexpected information about biological relationships.
- Provide the clinical story. Include the onset and progression of features, relevant examinations and previous test reports.
- Ask about missing samples. If a parent cannot participate, discuss singleton, duo or targeted family testing options and their limitations.
- Agree on follow-up. Who explains uncertain or negative results, and how can updated clinical information reach the laboratory?
A negative trio result can leave the genetic question open. Use the testing-limitations guide to distinguish an unmeasured mechanism from an unresolved interpretation. The secondary-findings guide can help with consent, and the care directory links to professional support.