It is an autosomal aneuploidy (Failure of segregation of chromatids during cell division cycle results in the gain or loss of a chromosome ).
Trisomy of 21 st chromosome. first described by Langdon Down (1866).
Main symptoms are :
individual is short statured,
Small round head,
Short neck,
Moon shaped face,
Mongolian eyelids.
Projecting lower lip.
Ever open mouth.
Furrowed Protruding tongue,
Palm is broad with characteristic palm crease.
Physical, psychomotor and mental development is retarded