Nonmosaic

Nonmosaic is a genetic term used to describe an organism or tissue in which all cells share the same genetic makeup. In a nonmosaic individual, the genome is uniform across different tissues, originating from a single fertilized egg with no post-zygotic genetic changes that create distinct cell lineages. This contrasts with mosaicism, where two or more genetically distinct cell populations exist within the same organism due to mutations or chromosomal abnormalities after fertilization.

The status of being nonmosaic depends on the extent and method of genetic testing. If the mutation or chromosomal alteration is present only in some cells, or at very low levels, sampling limitations may falsely suggest a nonmosaic state. Consequently, demonstrating nonmosaic status usually requires broad testing across multiple tissues or deep sequencing to rule out low-level mosaicism. Even when testing is extensive, absolute certainty can be challenging, since rare post-zygotic events may escape detection.

Clinically, nonmosaic interpretations often imply that a detected genetic change is present in all or most body cells, supporting conclusions about inheritance patterns, risk to offspring, and the rationale for diagnosing constitutional (system-wide) genetic conditions. By contrast, mosaic conditions may present with tissue-restricted manifestations or variable expressivity, reflecting the distribution of the mosaic cell lines. Related concepts include chromosomal mosaicism and chimerism, which involve mixtures of cell populations with different genetic compositions, though chimerism arises from the fusion of two zygotes rather than post-zygotic mutations within a single embryo.