3. Definition
Down syndrome (DS) or Down's syndrome, also
known as trisomy 21, is a chromosomal condition
caused by the presence of all or part of a third copy of
chromosome 21. Down syndrome is the most common
chromosome abnormality in humans.
4. Sign and symptoms
A small head compared to body size
Broad feet with short toes
Ears that are small and set low
Short arms and legs compared to length of body
Broad hands with short fingers and a single crease across the palm
Small nose and small mouth, in which the tongue may be relatively large
Lack of muscle tone
Ability to extend body joints; extreme flexibility
Eyes that slant upward and outward.
5. Pathogenesis
Nondisjunction is a faulty cell division that occurs before or at
conception when a pair of number 21 chromosomes
Mosaicism occurs after fertilization when nondisjunction of chromosome
21 takes place during an initial cell division. The result is that some of the
cells have 47 chromosomes
Translocation occurs either before or at conception and involves part of
chromosome 21 breaking off during cell division and attaching to another
