Body hair growth is modulated by a signalling molecule in the body called DHT. DHT is manufactured from testosterone. Although women have some testosterone, men have a hell of a lot more of it, so a lot more to convert into DHT. Women could theoretically grow bushy beards if for example a mutation greatly increased testosterone levels or caused a bug in DHT processing that made low levels of DHT cause a lot more hair growth than intended. This is why trans men can naturally grow beards just from hormone replacement therapy: More testosterone is added to the body, which means more DHT, even though the genes that would produce testosterone are much less active.
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