Scientists have found a clue to why one type of DNA is passed down to children by their mothers — but not their fathers. It is a very long-standing mystery in biology. Researchers report online June 23 in Science, DNA inside energy-producing organelles called mitochondria is destroyed in a dad’s sperm shortly after it fertilizes an egg. A protein called CPS-6 cuts apart the mitochondrial DNA in the male sperm so that the DNA can’t make the proteins that the mitochondria need to power the cell. Lingering paternal mitochondrial DNA might hurt developing embryos.