A small chromosome containing a centromere occasionally
seen in tissue culture, often in a mosaic state (present in some cells but
not in others). A marker chromosome may be of little clinical significance
or, if it contains material from one or both arms of another chromosome,
may create an imbalance for whatever genes are present; assessment to establish
clinical significance, particularly if found in a fetal karyotype, is often
difficult.
Definition from: GeneTests
from the University of Washington and Children's Health System, Seattle