TRISOMY 21

TRISOMY 21
The overall incidence is 1/800, but there is a marked variability depending on maternal ages: for
mothers in their 20s, the incidence is about 1/2000; for mothers >age 40, it rises to about 1/40 .
12th Frame: Un jeune souffrant de la tisomie 21 faisant l’instruction d’un plus petit ne souffrant pas de la
trisomie 21
Ordinairement, dans la trisomie 21, la face est ronde avec le meme contour que le calvaire,comme le visage de l’enfant
et pas aussi belle que la premiere photo du 12ieme encadrement; la premiere photo se refererait a une varaiante moins
severe que la deuxieme photo.
Down syndrome results when a person inherits all or part of an extra copy of
chromosome 21. This can occur in a variety of ways, the causes of which are unknown .
All the cells have the abnormality :
 The most common chromosomal abnormality that produces Down syndrome (accounting for
about 95 percent of all cases) is Trisomy 21, a defect in which an extra, third copy of
chromosome 21 is present in every cell in the body, which in 95% of cases is maternally
derived. The risk of Trisomy 21 is directly related to the age of the mother. (The number of Down
syndrome births is relatively low for 18-year-old mothers—about 1 in 2100 births. In the later childbearing years
the risk increases significantly—from 1 in 1000 births for 30-year-old women to 1 in 100 births for 40-year-old
women.)
 Two other chromosomal abnormalities cause Down syndrome and occur in about 2 to 3
percent of all cases.
 The first, translocation, takes place when a child inherits a small, extra piece of the
21st chromosome that is attached to another chromosome.
 t(14:21) is the most common in which the the additional chromosome 21 is
attached to a chromosome 14.
 In about half the cases, both parents will have normal karyotypes,
indicating a de novo trasnlocation in the affected child.
 In the other half, one parent (almost always the mother), although
phenotypically normal, has 45 chromosomes, one of which is
t(14:21). Theoritically, the chance for her to have a Down’s
syndrome child is 1/3, bu for unknown reasons the actual risk is
lower (about 1/10). If the father is the carrier, the risk is only 1/20
 t(21:22) is the next most common translocation. A carrier mother has
about a 1/10 risk of having a Down syndrome child, with an even smaller
risk for carrier fathers.
 t(21:21): In extreme;y rare cases, a parent carries the a t(21:21). In this
case, 100% of surviving offspring will be expected to have Down
syndrome
Mosaic Down syndrome results from a second type of chromosomal abnormality in which only
some cells in the body have an extra chromosome.
------------------------"Down Syndrome," Microsoft(R) Encarta(R) 98 Encyclopedia. (c) 1993-1997 Microsoft Corporation.
The Merck Manual; 17 th edition; pp. 2233-34