Trisomy

Trisomy is a case of aneuploidy in which there are three copies of a chromosome instead of in two in a diploid organism such as a human being.

The different trisomies

The majority of trisomies are not viable for the foetus which is not therefore born. However, there are viable trisomies named after the chromosomes which they affect:

• Trisomy 13 and trisomy 18 which often lead to death shortly after birth;
• trisomy 21 or Down's syndrome;
• and trisomy of the sex chromosomes.