Definition Down syndrome (DS) or Downs syndrome, also known as trisomy 21, is a chromosomal conditioncaused by the presence of all or part of a third copy ofchromosome 21. Down syndrome is the most common chromosome abnormality in humans.
Sign and symptomsA small head compared to body sizeBroad feet with short toesEars that are small and set lowShort arms and legs compared to length of bodyBroad hands with short fingers and a single crease across the palmSmall nose and small mouth, in which the tongue may be relatively largeLack of muscle toneAbility to extend body joints; extreme flexibilityEyes that slant upward and outward.
Pathogenesis Nondisjunction is a faulty cell division that occurs before or at conception when a pair of number 21 chromosomesMosaicism occurs after fertilization when nondisjunction of chromosome21 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 ofchromosome 21 breaking off during cell division and attaching to another