Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Trisomy 18 ?
    ### Response:
    Trisomy 18 is a chromosome disorder characterized by having 3 copies of chromosome 18 instead of the usual 2 copies. Signs and symptoms include severe intellectual disability; low birth weight; a small, abnormally shaped head; a small jaw and mouth; clenched fists with overlapping fingers; congenital heart defects; and various abnormalities of other organs.
    