Humans are born with 23 pairs of chromosomes, including two sex chromosomes, making a total of 46 in each cell.
People with Down's syndrome have three - rather than two - copies of chromosome 21.
![](https://sphotos-a.xx.fbcdn.net/hphotos-prn1/p480x480/942516_373843599387582_1388519814_n.png)
![Down's syndrome is caused by an extra chromosome](http://news.bbcimg.co.uk/media/images/68791000/jpg/_68791834_m1400332-down_s_syndrome_baby-spl.jpg)
Down's syndrome is caused by an extra chromosome
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