Image of the karyotype of a child with Down syndrome or Trisomy 21:

According to the NIH, 'Down syndrome is the most frequent genetic cause of mild to moderate mental retardation and associated medical problems and occurs in one out of 800 live births, in all races and economic groups. Down syndrome is a chromosomal disorder caused by an error in cell division that results in the presence of an additional third chromosome 21 or "trisomy 21."'

As you can see in the above karyotype, instead of the typical 46 chromosomes, children with Down syndrome usually have 47 chromosomes.

More Pictures:

• Health Pictures

Reproduced from the U.S. Department of Energy Human Genome Program.