What is a sign that suggests a true mosaic case from prenatal testing results?

A true mosaic case in prenatal testing refers to the presence of two or more genetically different cell lines within the same individual, which can occur due to various reasons during early development. The indication of multiple abnormal colonies with varying chromosome counts strongly suggests a mosaic situation because it reflects the existence of different populations of cells that have undergone distinct chromosomal alterations. This variability in chromosome counts aligns with the definition of mosaicism, as it demonstrates that not all cells share the same genetic composition.

In contrast, consistency in chromosome abnormality across all colonies would typically indicate a uniform genetic anomaly rather than mosaicism, as it suggests that a single aberration is present in all examined cells. Similarly, the detection of only one type of abnormality in all samples implies a monogenic issue rather than the diverse chromosomal scenarios seen in true mosaicism. Rapid cell growth in cultured samples can be a characteristic of certain cellular conditions but does not provide specific insights into the mosaics of chromosomal structure and counts. Therefore, the presence of multiple abnormal colonies with varying chromosome counts is the most telling sign of a true mosaic case in the context of prenatal testing.