Definition of "trisomy" [trisomy]

• The condition of having three copies of a given chromosome in each somatic cell rather than the normal number of two. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "The doctors soon discovered that he had multiple physical problems due to a genetic condition called trisomy 18."
• "The baby had a serious chromosomal problem called trisomy 18, and Helen had to terminate the pregnancy."
• "ROWLAND: Sam Strickland (ph) and Mandy Lemky's (ph) son was born with a deadly chromosomal disorder, known as trisomy 18, instead of two chromosome 18s, he had three."