Mitochondria are found abundantly in the midpiece of the sperm. The midpiece is located just behind the head of the sperm and contains many mitochondria that provide energy for the sperm to swim towards the egg. The egg, on the other hand, contains mitochondria throughout its cytoplasm. This is because the egg needs a large amount of energy to support the early stages of embryonic development, before the zygote can begin to produce its own energy.
During fertilization, the sperm and egg combine to form a zygote. The mitochondria from the sperm are typically destroyed after fertilization, so the zygote relies solely on the mitochondria from the egg for energy production. This is why mitochondrial DNA is inherited solely from the mother in most animals, including humans.