Originally Posted by
LordCdrMilitant
As I understand it, it's definitely possible for the opposite to be true [that there are female space marines but all have XY]. But in order for the genophage that modifies the person into a space marine to work, they have to have a Y chromosome. Presumably, it uses the fact that the Y-chromosome is always expressed [whereas, in AFAB people, one X chromosome is actually dormant and one is active] and tiny to add all the other space marine gene stuff to the end of the Y chromosome or something and guarantee that it's expressed while having reduced potential to malfunction due to interactions with also-active matching genes on active chromosomes.
OTOH, both statments are probably be true in the Emperor's Children.