Down's syndrome...

...Unfortunately, sometimes the process of gamete formation goes wrong. In one particular case, a gamete may carry two copies of the 21st human chromosomes. If this gamete is fertilized by a normal gamete, then the zygote which results will have three copies of chromosome 21. This results in the disorder known as Down's syndrome. Children with this condition show varying degrees of mental retardation, stunted physical growth, a reduced resistance to disease and congenital heart defects.....

*******a part of my course work.....
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