<prompt> Which of the following is the origin of the mitochondrial DNA of all human adult cells?Paternal onlyMaternal onlyA combination of paternal and maternalEither paternal or maternal  <response>: The mitochondrial DNA of all human adult cells is of maternal origin only. In human feilization, the entire sperm enters the secondary oocyte cytoplasm. However, sperm mitochondria degenerate along with the sperms tail. Therefore, only mitochondria present within the secondary oocyte maternal remain in the feilized zygote. 