Below is an instruction from Human. Write a response.
    ### Instruction:
    What are the genetic changes related to tetrasomy 18p ?
    ### Response:
    Tetrasomy 18p results from the presence of an abnormal extra chromosome, called an isochromosome 18p, in each cell. An isochromosome is a chromosome with two identical arms. Normal chromosomes have one long (q) arm and one short (p) arm, but isochromosomes have either two q arms or two p arms.
    