Cannabis males carry XY chromosomes and females XX, which is the reason regular seeds split in half. Feminized seed is made by forcing a female to produce pollen, so every seed gets an X from both sides and can only be female, while a true male passes on either an X or a Y. This is spelled out with a diagram on https://breederregularseeds.com, and it covers why environmental factors can nudge expression without changing the underlying chromosomes. Knowing this makes much of the breeding advice feel less like folklore.