Below is an instruction from Human. Write a response.
    ### Instruction:
    What are the genetic changes related to Down syndrome ?
    ### Response:
    Most cases of Down syndrome result from trisomy 21, which means each cell in the body has three copies of chromosome 21 instead of the usual two copies.  Less commonly, Down syndrome occurs when part of chromosome 21 becomes attached (translocated) to another chromosome during the formation of reproductive cells (eggs and sperm) in a parent or very early in fetal development.
    