Below is an instruction from Human. Write a response.
    ### Instruction:
    Is trisomy 18 inherited ?
    ### Response:
    Most cases of trisomy 18 are not inherited, but occur as random events during the formation of eggs and sperm. An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 18.
    