Down's syndrome (DS) is a  chromosomal disorder caused by the presence of all or part of an extra 21st  chromosome. It is named after John Langdon Down, the British physician who  described the syndrome in 1866. The disorder was identified as a chromosome 21  trisomy in 1959. The incidence of DS is estimated at 1 per 800 to 1,000 births,  although it is statistically much more common with older mothers.
             
           The most common  manifestations of DS are the characteristic facial features, cognitive  impairment, congenital heart disease, hearing deficits, short stature, thyroid  disorders, and Alzheimer's disease. Other less common serious illnesses include  leukaemia, immune deficiencies, and epilepsy.  
           Individuals with DS tend to  have a lower than average cognitive ability, often ranging from mild to  moderate developmental disabilities. A small number have severe to profound  mental disability. The plus side of this chromosomal disorder, on the other  hand, is a great emotional depth characteristic of people with DS.  
          Read more about DS here (Wikipedia).   
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