trisomy

n., pl., -mies.
The condition of having three copies of a given chromosome in each somatic cell rather than the normal number of two.

[TRI- + -SOM(E)³ + -Y².]

trisome tri'some' n.
trisomic tri·so'mic adj.

(trī′səmē)
n

An additional chromosome in the normal complement, so that in each nucleus a chromosome is represented three times rather than twice.

The presence of an additional (third) chromosome of one type in an otherwise diploid cell (2n + 1); associated with many congenital deformities in humans.