blt

The gene is on X chromosome, not on Y chromosome, so it can't be the reason.

I know it is very rare situation. But what is the genetic reason of lethality of XhXh? If mutation changes life-important gene at X chromosome, then XhY- should also be lethal. But it isn't. Many males live quite normally, and some of them even don't know they are sick. It's easy to understand why some mutations on X chromosome are lethal for males and aren't for females. Females have second X chromosome where healthy gene exists. This rescues females lives in many recesive X-linked diseases. But hemophilia is also recesive X-linked disease and situation is opposite. It's often not lethal for hemizygotic males (XhY-), where there is no chance for healthy gene, because there is no second X chromosome. It's lethal for homozygotic females (XhXh), where there is also no chance for healthy gene. Why it is like that?

In prenatal stage, as I heard.

Why hemophilia is lethal (in almost 100%) for homozygotic females, while it's not lethal for hemizygotic males? Both, homozygotic females and hemizygotic males don't have any healthy allel. Thank you for replies