Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Down syndrome ?
    ### Response:
    What causes Down syndrome? There are 3 possible genetic causes of Down syndrome: Trisomy 21. Most often, Down syndrome is caused by an extra chromosome 21 in all cells of the affected person.  In these cases, the chromosome 21 pair fails to separate during the formation of an egg (or sperm); this is called "nondisjunction.
    