Additional derivative chromosome 21

The metaphase on the left and the karyotype on
the right show a small additional chromosome
(arrow) with fluorescence in two colors.
Detailed analysis of many metaphases revealed
a compound chromosome consisting of material
from a chromosome 18 and a centromere
from a chromosome 21. Thus, the karyotype is
unbalanced for additional material from a chromosome
18 (partial trisomy 18), which is a
cause of developmental retardation. The origin
of such a compound chromosome usually remains
obscure.