Most of the DNA changes that cause an inherited condition are passed down the family tree, copy by copy, from a parent who carries the same change. A de novo variant works differently. It is a change in a gene that shows up for the first time in a child and is not present in either parent's DNA. The term "de novo" simply means "new."
Where a de novo variant comes from
A new variant usually arises in one of two ways. It can occur in an egg or sperm cell before conception, so the change is already present in the fertilized egg even though neither parent carries it in their own body cells. Or it can occur very early in embryonic development, after the egg and sperm have already joined, so the change ends up in some or all of the cells of the growing embryo. Because the change happens during the formation of reproductive cells or in the earliest stages of development, it is often impossible to pin down exactly when it happened.
This is a different process from the autosomal dominant and autosomal recessive patterns, and from X-linked inheritance, which all describe how an already-existing variant travels from parent to child. A de novo variant does not travel from anyone. It appears new, and from that point on it can be passed to the next generation the way any other variant would be.
Why this matters for families
For a family, the most immediate effect of a de novo cause is that a genetic condition can appear with no warning and no family history at all. Parents are sometimes surprised, or doubt a diagnosis, because no one else in either family tree has had anything similar. A de novo origin is a well-documented and fairly common explanation for exactly that situation.
The recurrence question, whether a future child of the same two parents could be affected again, usually has a reassuring answer when a variant is confirmed to be de novo. Since neither parent carries the variant in their own egg or sperm cells in the ordinary case, the chance that it would happen a second time in the same family is generally low, closer to the background chance in the general population than to the higher recurrence risk seen with many inherited patterns.
There is an important exception worth knowing about: germline mosaicism. In this situation, a variant is present in some, but not all, of a parent's egg or sperm cells, while the rest of that parent's body is unaffected and tests as having no variant at all. Because standard testing of blood or saliva reflects the parent's body cells, germline mosaicism can be easy to miss, and a parent can pass a condition to more than one child even though they themselves show no signs of it and no variant turns up on their own testing. This is one of the reasons a genetic counselor talks through recurrence risk carefully rather than treating "de novo" as an automatic guarantee against recurrence. For a personalized recurrence estimate, genetic testing and counseling is the right next step, not a general reference page.
A documented mechanism across several conditions
De novo variants are not a rare curiosity. They are a recognized cause across a range of the conditions covered on this site. In Rett syndrome, the great majority of cases trace back to a new variant in a single gene, with no history of the condition anywhere else in the family. In Angelman syndrome, some cases arise from a new genetic change affecting the maternally inherited copy of the relevant gene region, occurring around the time reproductive cells form or in very early development; other cases of Angelman syndrome involve a related mechanism called genomic imprinting, where it matters which parent a gene copy came from, not only whether a variant is new or inherited. In tuberous sclerosis complex, a substantial share of cases are also due to a new variant arising in a person with no family history of the condition, alongside other cases that are clearly inherited from an affected parent.
Seeing the same mechanism turn up across unrelated conditions is a useful reminder that a de novo finding on a genetic test is not itself a diagnosis of any one condition. It is a statement about where a specific variant came from. The condition it causes, and what that means day to day, depends on which gene is affected and how.
This page is educational and is not a tool for self-diagnosis. A genetic test result and its family implications are best reviewed with a clinician or a genetic counselor who can look at the specific gene, the specific variant, and the family's own history.