Male Down's syndrome karyotype, artowrk. The complete set of chromosomes of an organism is known as the karyotype. Humans usually have 46 chromosomes in total, arranged into 23 pairs. Down's syndrome, also called trisomy 21, is caused by the presence of three copies of chromosome 21 (bottom left). Down's syndrome results in mental retardation and flattened facial features. The XY chromosome pair at bottom centre indicates that this person is female. Females have an XX chromosome pair instead.