A child is born with 46 chromosomes but a child with extra copy of one of these chromosomes, chromosome 21 and in medical term for having extra copy of chromosome is trisomy , also known as down syndrome.
Down syndrome which includes the low muscle tone, flat nasal bridge, protruding tongue. It have high risk of some conditions which includes Alzheimer's disease and epilepsy. 95 % times down syndrome is caused by trisomy 21 when the person has three copies of chromosome 21 which instead of usual two copies in all cells.
The presence of three copies of chromosome is causes primary down syndrome. Half the chromosomes come from the egg and half come from the the father. This XY chromosomes form the egg and Y chromosomes from the father.
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