Hey. Remember when Valve essentially confirmed that Zoey and Rochelle are men?
Since the only chromosome that pertains exclusively to the father’s side (and can therefore only be passed down from the father’s side) is the Y chromosome, that means the carrier gene must reside on the Y chromosome. So if someone is to be immune to the virus, they need to have a Y chromosome with the carrier gene. If somebody has a Y chromosome, well, you know the rest.