Definition - What does Trisomy mean?

Trisomy is a condition where there are three copies of a chromosome in a cell instead of the usual two copies. A chromosome is the unit of genetic information in a human cell. Humans usually have twenty-three pairs of chromosomes, giving a total of forty-six chromosomes. Where trisomy occurs, there are forty-seven chromosomes present. For any given pair of chromosomes, if trisomy occurs there will be three copies of the chromosome instead of two. This is a type of aneuploidy, meaning that there are an abnormal number of chromosomes. Aneuploidy can result in miscarriage or congenital abnormalities.

FertilitySmarts explains Trisomy

It is possible for trisomies to occur with any chromosome. The impact of the aneuploidy depends on the chromosome affected. It is possible for trisomy pregnancies to result in miscarriage or to be associated with conditions causing physical abnormalities or intellectual disability. Examples of trisomies include:

  • Trisomy 13, or Patau syndrome
  • Trisomy 16 (which most commonly leads to miscarriage)
  • Trisomy 18, or Edwards syndrome
  • Trisomy 21, or Down syndrome
  • XXX, or Triple X syndrome
  • XXY, or Klinefelter syndrome

Risk factors for trisomy include advanced maternal age.

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