Light Micrograph (LM) karyotype for a human male with Down Syndrome. Down syndrome (DS or DNS), also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is typically associated with physical growth delays, characteristic facial features and mild to moderate intellectual disability The average IQ of a young adult with Down syndrome is 50, equivalent to the mental ability of an 8-10 year-old child, but this can vary widely. Karyotypes are an arrangement of chromosomes based on their appearance under a light microscope. Magnification: unknown.