The Gold Standard of Genetics | De Novo Variants, Parental Mosaicism, Recurrence Risk and Family Testing

THE GOLD STANDARD OF GENETICS · SPECIALIST STUDY

Treat “de novo” as a tested inheritance claim, not a permanent label

A variant may appear new in a child because it arose in an egg, sperm or early embryo, because a parent carries it at a level missed in the tested tissue, because the family relationship or sample identity is wrong, or because the assay cannot see the relevant form of variation. The useful question is therefore not simply whether a report says de novo. It is how the child and parents were tested, what tissues and detection limits apply, whether parent of origin can be resolved, how phenotype and mechanism fit, and which evidence can make a family recurrence estimate more personal without pretending it is exact.

This guide belongs to the Genetics subject root, the Gold Standard of Education, and the How X Works hub. It begins with a concrete decision, then tests whether the method still holds when the context changes.

The whole problem at a glance

Each row changes the meaning of the next; a recurrence estimate is only as sound as the inheritance evidence beneath it.

CHAPTER 2 OF 13 · Define and verify the inheritance claim

2. Verify identity, relationship and specimen provenance

Confirm that the intended people and samples form the tested trio before drawing inheritance conclusions. Use documented relationship analysis when required rather than treating names on tubes as proof. The working unit here is one family, one verified molecular finding, specified relatives and tissues, and one reproductive or clinical question at a stated time. State whose sample, memory, decision or record is being examined, what time boundary applies and which claim is still outside the evidence.

Worked example: A sample swap is suspected because the genotype sharing pattern is inconsistent. The inheritance claim pauses until accession, consent and relationship evidence are reconciled. Put the starting information, decision, later observation and consequence on separate lines. That chronology stops a coherent story from replacing a reproducible account.

Evidence to retain: consent; accession and barcode; collection time; chain; sex-chromosome and kinship checks where appropriate; contamination metrics; recollection history Attach dates, versions, provenance, custody and the decision use. Record unavailable evidence explicitly; absence is not a negative result and cannot safely be turned into reassurance.

Boundary and misconception: A sophisticated variant caller cannot repair the wrong family structure or a mislabeled specimen. Test a credible competing explanation and name what evidence would change the conclusion. The pass condition is a bounded, reviewable claim, not a promise that the method removes uncertainty.

Evidence discipline: record the observation, version or time boundary that supports this chapter’s conclusion. Preserve the rejected alternative as well as the selected action. That makes a later review able to distinguish a changed world from an undocumented change of mind.

Changed-context transfer: Move collection to separate clinics and design a provenance check that survives the handoff. The method transfers only if the decision question and measurement meaning survive the change. If either changes, rebuild the comparison rather than copying the earlier threshold or weight.

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CHAPTER 4 OF 13 · Find mosaicism and model recurrence

4. Read parental non-detection through the assay limit

Describe what parental testing could detect in that specimen and region. A negative result is bounded by coverage, read depth, allele fraction, copy-number sensitivity and mapping quality. The working unit here is one family, one verified molecular finding, specified relatives and tissues, and one reproductive or clinical question at a stated time. State whose sample, memory, decision or record is being examined, what time boundary applies and which claim is still outside the evidence.

Worked example: Both parents test negative on a standard panel, but the report does not quantify low-level detection. The team requests method details before using the result in counselling. Put the starting information, decision, later observation and consequence on separate lines. That chronology stops a coherent story from replacing a reproducible account.

Evidence to retain: parental raw data; target coverage; validated detection threshold; variant-specific interference; specimen quality; confirmation result Attach dates, versions, provenance, custody and the decision use. Record unavailable evidence explicitly; absence is not a negative result and cannot safely be turned into reassurance.

Boundary and misconception: “Not detected” is not “not present anywhere in the body or germline.” Test a credible competing explanation and name what evidence would change the conclusion. The pass condition is a bounded, reviewable claim, not a promise that the method removes uncertainty.

Evidence discipline: record the observation, version or time boundary that supports this chapter’s conclusion. Preserve the rejected alternative as well as the selected action. That makes a later review able to distinguish a changed world from an undocumented change of mind.

Changed-context transfer: Reduce parental allele fraction below the validated threshold and state how the recurrence discussion changes. The method transfers only if the decision question and measurement meaning survive the change. If either changes, rebuild the comparison rather than copying the earlier threshold or weight.

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CHAPTER 6 OF 13 · Work the family-testing case

6. Model recurrence as a distribution that evidence can update

Begin with the mechanism and empirical evidence, then update with parental age, mosaicism testing, parent of origin and family history. Present ranges and residual uncertainty, not false precision. The working unit here is one family, one verified molecular finding, specified relatives and tissues, and one reproductive or clinical question at a stated time. State whose sample, memory, decision or record is being examined, what time boundary applies and which claim is still outside the evidence.

Worked example: A family receives a broad baseline estimate after one affected child. Deep parental testing later finds low-level mosaicism, so the estimate is revised upward with an explanation of remaining sampling uncertainty. Put the starting information, decision, later observation and consequence on separate lines. That chronology stops a coherent story from replacing a reproducible account.

Evidence to retain: gene and mechanism; variant class; parental testing sensitivity; mosaic signal; affected siblings; reproductive history; study population; confidence interval Attach dates, versions, provenance, custody and the decision use. Record unavailable evidence explicitly; absence is not a negative result and cannot safely be turned into reassurance.

Boundary and misconception: The familiar “about one percent” shorthand is not a universal law and may be too high or too low for a particular mechanism and family. Test a credible competing explanation and name what evidence would change the conclusion. The pass condition is a bounded, reviewable claim, not a promise that the method removes uncertainty.

Evidence discipline: record the observation, version or time boundary that supports this chapter’s conclusion. Preserve the rejected alternative as well as the selected action. That makes a later review able to distinguish a changed world from an undocumented change of mind.

Changed-context transfer: Add a second affected pregnancy and show why the prior probability and testing strategy must change. The method transfers only if the decision question and measurement meaning survive the change. If either changes, rebuild the comparison rather than copying the earlier threshold or weight.

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CHAPTER 8 OF 13 · Work the family-testing case

8. Work a complete family case without forcing certainty

Join the child’s phenotype, confirmed variant, trio evidence, parental tissue testing, phasing and counselling choices in one reviewable chronology. The working unit here is one family, one verified molecular finding, specified relatives and tissues, and one reproductive or clinical question at a stated time. State whose sample, memory, decision or record is being examined, what time boundary applies and which claim is still outside the evidence.

Worked example: A child has a confirmed pathogenic variant absent from routine parental blood. Read-backed phasing points to the father; deep targeted testing detects a low signal in blood and sperm. The family receives a revised, bounded risk discussion and available testing choices. Put the starting information, decision, later observation and consequence on separate lines. That chronology stops a coherent story from replacing a reproducible account.

Evidence to retain: phenotype and diagnosis; classification evidence; trio QC; phase; tissue results; detection limits; counselling record; chosen follow-up Attach dates, versions, provenance, custody and the decision use. Record unavailable evidence explicitly; absence is not a negative result and cannot safely be turned into reassurance.

Boundary and misconception: The fictional case illustrates evidence updating. It does not supply a personal risk percentage or imply that sperm testing is appropriate, available or decisive in every family. Test a credible competing explanation and name what evidence would change the conclusion. The pass condition is a bounded, reviewable claim, not a promise that the method removes uncertainty.

Evidence discipline: record the observation, version or time boundary that supports this chapter’s conclusion. Preserve the rejected alternative as well as the selected action. That makes a later review able to distinguish a changed world from an undocumented change of mind.

Changed-context transfer: Change the finding to a maternally derived variant with reduced penetrance and rebuild the counselling question. The method transfers only if the decision question and measurement meaning survive the change. If either changes, rebuild the comparison rather than copying the earlier threshold or weight.

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Family-testing decision gate

Declining further testing is also a decision that deserves accurate documentation without coercion.

CHAPTER 10 OF 13 · Counsel, document and revisit

10. Discuss reproductive options without converting counselling into direction

Explain prenatal diagnosis, preimplantation testing, donor options, natural conception and declining testing in neutral language, including technical and timing limits. The working unit here is one family, one verified molecular finding, specified relatives and tissues, and one reproductive or clinical question at a stated time. State whose sample, memory, decision or record is being examined, what time boundary applies and which claim is still outside the evidence.

Worked example: Parents compare targeted prenatal diagnosis with conception without testing. The counsellor records their values and avoids presenting one path as the responsible choice. Put the starting information, decision, later observation and consequence on separate lines. That chronology stops a coherent story from replacing a reproducible account.

Evidence to retain: confirmed familial variant; laboratory acceptance; timing; procedure limits; residual risk; patient preferences; interpreter; follow-up owner Attach dates, versions, provenance, custody and the decision use. Record unavailable evidence explicitly; absence is not a negative result and cannot safely be turned into reassurance.

Boundary and misconception: Availability, law, cost and values differ. Genetic evidence should support informed choice, not predetermine it. Test a credible competing explanation and name what evidence would change the conclusion. The pass condition is a bounded, reviewable claim, not a promise that the method removes uncertainty.

Evidence discipline: record the observation, version or time boundary that supports this chapter’s conclusion. Preserve the rejected alternative as well as the selected action. That makes a later review able to distinguish a changed world from an undocumented change of mind.

Changed-context transfer: Move the family to a setting where one option is unavailable and redesign an honest set of choices. The method transfers only if the decision question and measurement meaning survive the change. If either changes, rebuild the comparison rather than copying the earlier threshold or weight.

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CHAPTER 12 OF 13 · Transfer the method and state its limits

12. Plan reanalysis, recontact and result correction

Store the variant, transcript, reference build, classification, inheritance evidence and counselling decision with dates so future reinterpretation can find the right family. The working unit here is one family, one verified molecular finding, specified relatives and tissues, and one reproductive or clinical question at a stated time. State whose sample, memory, decision or record is being examined, what time boundary applies and which claim is still outside the evidence.

Worked example: A laboratory improves low-level mosaic detection two years later. The service identifies eligible families, confirms consent and current contact pathways, and offers review without promising a changed answer. Put the starting information, decision, later observation and consequence on separate lines. That chronology stops a coherent story from replacing a reproducible account.

Evidence to retain: versioned report; raw-data retention; consent and recontact preference; laboratory updates; affected-relative log; correction and notification record Attach dates, versions, provenance, custody and the decision use. Record unavailable evidence explicitly; absence is not a negative result and cannot safely be turned into reassurance.

Boundary and misconception: Recontact duties vary and cannot depend on an informal promise that one clinician will remember every family indefinitely. Test a credible competing explanation and name what evidence would change the conclusion. The pass condition is a bounded, reviewable claim, not a promise that the method removes uncertainty.

Evidence discipline: record the observation, version or time boundary that supports this chapter’s conclusion. Preserve the rejected alternative as well as the selected action. That makes a later review able to distinguish a changed world from an undocumented change of mind.

Changed-context transfer: Transfer care to another system and test whether the evidence and recontact trigger remain understandable. The method transfers only if the decision question and measurement meaning survive the change. If either changes, rebuild the comparison rather than copying the earlier threshold or weight.

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Limits and misconceptions

De novo is a tested inheritance claim

It is bounded by the trio, tissues, method, identity checks and detection limit.

Parental blood is not the whole parent

Tissue-restricted or very low-level mosaicism may remain undetected.

Recurrence risk is not one universal percentage

Mechanism, mosaicism evidence and family history update a distribution.

Family testing needs a decision and consent

A useful test answers a defined question and protects relatives’ autonomy.

Practise, verify and continue

Create a one-page family evidence map with the variant, child phenotype, trio identity, parental specimens, assay limits, phase, mosaicism evidence, recurrence rationale, tested relatives and recontact trigger. Add one alternative explanation that remains possible.

Apply the same method to an apparently de novo copy-number variant. Rebuild assay sensitivity, breakpoint confirmation, parental balanced rearrangement testing and recurrence reasoning rather than reusing the single-nucleotide-variant pathway.

Practical evidence check

Before implementation, write a one-page evidence map with the decision, unit of analysis, version or time boundary, inputs, outcomes, uncertainty, authority and stop rule. Attach one worked case that passes and one that fails. A reviewer should be able to reproduce the reasoning without relying on the memory of the person who built the system.

Run one changed-context rehearsal before approval. Alter a mechanism rather than a cosmetic detail, then identify which calculation, safeguard or governance gate must change. Record any conclusion that does not transfer. This short rehearsal is often the fastest way to expose a hidden assumption before it becomes an operational dependency.

Related eduKate reading

Mosaicism, variant allele fractions and tissue-specific detection
Identity-by-descent, phasing errors and kinship inference
Reverse phenotyping, allelic series and genotype-first recall

Authoritative sources and scope

GeneReviews: Resources for Genetics Professionals — Mosaicism
Parental mosaicism and recurrence risk review
Personalised recurrence-risk assessment after de novo pathogenic variants
Primary study of parental germline mosaicism and recurrence

These sources support the visible distinctions and evidence practices described here. They do not replace patient-specific genetic counselling, laboratory validation, consent, local regulation or a clinician’s interpretation. All worked cases are fictional, recurrence figures are illustrative rather than personal estimates, and no reader should start, stop or change medical care from this educational article.

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