Numerical Abnormalities: Overview of Trisomies and Monosomies

What are numerical chromosome abnormalities?

Numerical abnormalities are a type of chromosome defect. These types of birth defects occur when there is a different number of chromosomes in the cells of the body from what is usually found. So, instead of the typical 46 chromosomes in each cell of the body, there may be 45 or 47 chromosomes. Having too many or too few chromosomes may cause health problems or birth defects.

What are trisomies?

The term "trisomy" is used to describe the presence of 3 chromosomes, rather than the usual pair of chromosomes. For example, if your baby is born with 3 #21 chromosomes, rather than the usual pair, your baby would be said to have "trisomy 21." Trisomy 21 causes Down syndrome. Other examples of trisomy include trisomy 18 and trisomy 13. Again, trisomy 18 or trisomy 13 simply means there are 3 copies of the #18 chromosome (or of the #13 chromosome) present in each 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 is a total of 45 chromosomes in each cell of the body, rather than 46. For example, if your baby is born with only one X chromosome, rather than the usual pair (either two X's or one X and one Y chromosome), your baby would be said to have "monosomy X." Monosomy or partial monosomy is the cause of certain diseases such as Turner syndrome and Cri-du-Chat syndrome.