Down syndrome is a chromosomal abnormality characterized by the presence of an extra copy of genetic material on the 21st chromosome, either in whole or part.
Chromosome 21 - How accidental inheritance can lead to Downs syndrome by The Royal Institution
The effects of the extra copy varies greatly from individual to individual, depending on the extent of the extra copy, genetic background, environmental factors, and random chance.
Down syndrome (trisomy 21) - causes, symptoms, diagnosis, & pathology by Osmosis
Down syndrome can occur in all human populations, and analogous effects have been found in other species, such as chimpanzees and mice.
Recently, researchers have been able to create transgenic mice with most of human chromosome 21.
A transgene is a gene or genetic material that has been transferred naturally, or by any of a number of genetic engineering techniques from one organism to another.
In the bottom right, there are chromosomal differences between males and females, which do not concern us.
A typical human karyotype is designated as 46,XX or 46,XY, indicating 46 chromosomes with an XX arrangement for females and 46 chromosomes with an XY arrangement for males.