trisomy

(trī-sō'mē, trī'sō'-)
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.

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1. describing a cell, tissue, or organism having three copies of one or more chromosome(s) in an otherwise diploid chromosome set. In humans, trisomy 21 (i.e. presence of three copies of chromosome 21) is asssociated with Down's syndrome.
2. such a cell, tissue, or organism.

—trisomy n.

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

(trī′səmē)
n

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