Briefly explain Down's syndrome.

(N/A) Down's syndrome is a chromosomal disorder caused by the presence of an additional copy of chromosome number $21$ (trisomy of $21$).
This results in a total of $47$ chromosomes instead of the normal $46$.
This disorder was first described by Langdon Down in $1866$.
Key features of an affected individual include:
$1$. Short stature with a small round head.
$2$. Furrowed tongue and partially open mouth.
$3$. Palm is broad with a characteristic palm crease.
$4$. Physical,psychomotor,and mental development is retarded.
$5$. Eyes have epicanthic folds.
$6$. Congenital heart defects and underdeveloped reproductive organs.
The incidence of this syndrome is approximately $1$ in $700$ births,and it is more common in children born to mothers of advanced maternal age.