Trisomy 21

Trisomy 21, also called Downs syndrome, is a specific trisomy involving chromosome 21. It is one of the rare trisomies which is consistent with pregnancy being continued to term.

Consequences of trisomy 21

The child, however, has typical symptoms:

• a specific morphology (facies, short height) ;
• organ and joint malformations;
• and mental retardation which, however, does not often prevent social integration.

Although the clinical signs are well known the diagnosis is only made by performing a karyotype (analysis of the number of chromosomes in a cell).

Free trisomy and translocation trisomy

There are two forms of trisomy 21 : free trisomy and translocation trisomy.

• Free trisomy may arise from a gamete (ovum or spermatozoon) Incorrectly containing 24 chromosomes instead of 23. If all of the cells have 47 chromosomes, the term homogeneous trisomy 21 is used. If the cells have 46 chromosomes the term trisomy mosaic is used;
• Translocation trisomy 21 is rarer and involves the attachment of the third chromosome 21 to another chromosome. In this case there is a total of 46 chromosomes but in reality the genetic material of chromosome 21 is carried three times.