Below is an instruction from Human. Write a response.
    ### Instruction:
    What are the genetic changes related to trisomy 18 ?
    ### Response:
    Most cases of trisomy 18 result from having three copies of chromosome 18 in each cell in the body instead of the usual two copies. The extra genetic material disrupts the normal course of development, causing the characteristic features of trisomy 18.  Approximately 5 percent of people with trisomy 18 have an extra copy of chromosome 18 in only some of the body's cells.
    