What characteristic defines a mosaic chromosome abnormality?

A mosaic chromosome abnormality is characterized by the presence of two or more genetically different cell lines within an individual. This means that the abnormality is not uniformly present across all cells; rather, it exists in some cell lines while others may have a normal karyotype. For example, in conditions such as Turner syndrome (45,X/46,XX mosaicism), some cells have the typical chromosome number while others have a missing X chromosome.

This differentiates mosaicism from more uniform chromosomal abnormalities where the abnormality either affects all cells or does not occur at all. Hence, the correct choice reflects that the mosaic chromosome abnormality is not present in all cell lines, highlighting the variability that defines mosaicism in chromosomal patterns.