For a long time, biologists thought our DNA resided only in the control center of our cells, the nucleus.

Then, in 1963, a couple at Stockholm University discovered DNA outside the nucleus. Looking through an electron microscope, Margit and Sylvan Nass noticed DNA fibers in structures called mitochondria, the energy centers of our cells.

Our mitochondrial DNA accounts for a small portion of our total DNA. It contains just 37 of the 20,000 to 25,000 protein-coding genes in our body. But it is notably distinct from DNA in the nucleus. Unlike nuclear DNA, which comes from both parents, mitochondrial DNA comes only from the mother.

Nobody fully understands why or how fathers’ mitochondrial DNA gets wiped from cells. An international team of scientists recently studied mitochondria in the sperm of a roundworm called C. elegans to find answers.