KidsMD Health Topics

Trisomies and Monosomies

  • Overview

    A trisomy and a monosomy are types of numerical chromosome abnormalities that can cause certain birth defects. Normally, people are born with 23 chromosome pairs, or 46 chromosomes, in each cell — one inherited from the mother and one from the father. A numerical chromosome abnormality can cause each cell to have 45 or 47 chromosomes in each cell.

    What are trisomies?

    The term "trisomy" is used to describe the presence of an extra chromosome — or three instead of the usual pair. For example, trisomy 21 or Down syndrome occurs when a baby is born with three #21 chromosomes. In trisomy 18, there are three copies of chromosome #18 in every cell of the body, rather than the usual pair.

    What are monosomies?

    The term "monosomy" is used to describe the absence of one member of a pair of chromosomes. Therefore, there are 45 chromosomes in each cell of the body instead of the usual 46. Monosomy X, or Turner syndrome, occurs when a baby is born with only one X sex chromosome, rather than the usual pair (either two Xs or one X and one Y sex chromosome).


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    Boston Children's Hospital 
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