All About Trisomy 21 (Mongolism Or Down Syndrome)

Trisomy 21 or Down syndrome is a congenital malformation. It is due to the presence of a supernumerary chromosome on the 21st pair of chromosomes that is to say that instead of having a total of 46 chromosomes, the person carrying trisomy 21 has 47. The cause of this affection is unknown. The psychomotor development of people with Down syndrome is generally slowed down. The child suffering from Down Syndrome is referred to a Medical-Pedagogical Institute (IMP) then a Medical-Professional Institute (IMPro).

Trisomy 21: Definition

Trisomy 21 or Down syndrome is a congenital malformation. It is due to the presence of a supernumerary chromosome on the 21st pair of chromosomes that is to say that instead of having a total of 46 chromosomes, the person carrying trisomy 21 has 47 chromosomes. Ordinary men and women have 23 pairs of chromosomes, with two X chromosomes in women and one X and one Y in men.

The frequency of trisomy 21 is 1/700 births. Boys are as affected as girls. The actual frequency is much higher, but six embryos or fetuses out of 7 carriers of trisomy 21 die in utero ( spontaneous abortion ).

Causes And Risk Factors Of Trisomy 21

The cause of this condition is unknown.

Meiosis could give following a non-disjunction of unknown cause, two germ cells (oocytes): one with 24 chromosomes (two chromosomes 21 instead of one) and one with 22 chromosomes (no chromosome 21). If the ovum with two chromosomes 21 is fertilized by a normal spermatozoon, it leads to an embryo carrying three chromosomes 21.

The risk increases with the mother’s age of misdistribution of chromosomes during meiosis is large: 1/100 after 40 years and 1/46 after 45 years. Since many more mothers have children between the ages of 20 and 25 than between the ages of 40 and 45, it is not uncommon to meet children with Down’s syndrome of young mothers. For all women at risk (senior woman, trisomy in the family, etc.), amniocentesis allows prenatal diagnosis and the possibility of miscarriage.

In 95% of cases, it is a free chromosome, in 3% of cases, it is a translocation, and in 2% of cases, a mosaic.

A screening test for trisomy 21 exists, but the reliability of this first generation test was not 100%. A new high-throughput sequencing test with performance close to 100% has been available since 2015. This test is used to analyze circulating fetal DNA via DNA fragments from the fetus, present in maternal blood throughout pregnancy. This test should make it possible to avoid amniocentesis, but studies are continuing in France and abroad to validate the performance of this test.