tri·so·my 
(trī-sōmē, trīsō′-)
(trī-sōmē, trīsō′-)n. pl. tri·so·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′ n.
tri·somic adj.