This picture of Freddie's chromosomes was kindly provided by Claire Faulkner, a Geneticist at the Bristol Children's' hospital in the UK. It is the result of a chromosome test done shortly after he was born.
The chromosome pairs are numbered from 1 (the biggest) to 22 (the smallest). The 23rd pair are the sex chromosomes XX in females and XY in males. Normally, we have 46 chromosomes in every cell in our body, arranged into 23 matching pairs. Freddie has the classic and most common form of Downs known as Trisomy 21 (ie "three 21s). As you can clearly see, he has a whole extra copy of all the genes on chromosome 21, giving him three chromosome 21s instead of the normal two. No one knows exactly what effect this has, but it will effect the way that he grows and develops, and gives rise to the characteristic traits of the syndrome.