Board Thread:Warhammer 40k General Discussion/@comment-7699236-20150618150629/@comment-26516936-20150622083005

None of the 20 primarchs was female but what I was speculating about was a female primarch created at the same time the Emperor created the Lion but he decided not to gestate her beyond embryo status & put her in cold storage. Now that idea could easily be added to the canon without any difficulty or any major changes, at which point a huge array of story possibilities opens up. As for creating female astartes you can't from the current crops of geneseed as has been stated they are keyed for males & no one apart from the Emperor has the ability to modify geneseed to make female astartes possible. However creating new geneseed from a new female primarch, though incredibly difficult, would be just within the realms possibility. So in the end if we are to see female astartes they almost certainly would have to come on the heels of the introduction of a female primarch.