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a graphic showing lab grown egg and sperm

We all know the drill for reproduction—sperm meets egg.

For the past decade, scientists have been pushing the boundaries of where the two halves come from. Thanks to induced pluripotent stem cell technology, it’s now possible to scrape skin cells from mice and transform them into functional sperm or egg cells that give rise to healthy pups born from two moms or dads. The recipe may even open the door for single parent offspring—at least for mice.

But mice aren’t people. And the same recipe doesn’t work for human reproductive cells.

One reason, according to Dr. Mitinori Saitou at Kyoto University, is due to the chemical “tags,” known collectively as the epigenome, that control when certain genes are turned on or off. Like a ledger, these tags maintain a sort of memory in early reproductive cells. For cells to eventually develop into sperm or eggs, those memories must be wiped clean.

This month, Saitou’s team developed a recipe to do just that. Starting from primordial germ cells—a type of cell that eventually develops into either sperm or egg—they added...