chromosomes - (Mar/04/2007 )

I remember my teacher told me that there were 46 choromosomes in every cell. So here is my question: Down syndrome is a genetic disorder caused by the presence of all or part of an extra 21st chromosome. why here it says EXTRA 21st chromosome? i don't understand. There is supposed to be 46 choromosomes? Hope you can help me, thanks.

Human somatic cells have 23 pairs of chromosomes, one of each pair comes from the mother and one from the father. So 23 x 2 = 46 in total (germ cells, sperm and eggs, have only one of each pair, so 23 in total). People with down's syndrome have one extra chromosome 21, so 46 + 1 = 47.