A woman with 47 chromosomes due to three copies of chromosome 21 is...

Down's syndrome is due to an extra chromosome, which combines to the chromosome pair 21. Down syndrome is caused due to trisomy, in which the fusion of a normal $n$ gamete (haploid) takes place with an $n+1$ gamete (diploid). J. Langdon Down first described the developmental defect produced by trisomy in 1866. For this reason it is called Down's syndrome. Persons having Down's syndrome generally have a short, stocky build short hands, flattened facial features and poor muscle tone. In addition they are mentally retarded.