In a system of absolute primogeniture which does not take sex into account, a female heir apparent is not surprising. However, females are rarely heirs apparent to titles which follow male-preference primogeniture. A female can be heir apparent to such title if her father was the heir apparent who died leaving no sons. In such rare circumstances, that female would replace her father as heir apparent to whatever throne or title is concerned.
Special exceptions are Mary II of England and Anne, Queen of Great Britain. The former reigned jointly with her husband William and was also heir apparent to her husband (should he die before her, she would become sole monarch). However, William outlived Mary. The Act of Settlement 1701 determined that William would be succeeded by his sister-in-law and cousin, Anne, who was thus heir apparent until his death.
When succession follows matrilineal primogeniture, only females are entitled to inherit the throne and thus only females can be heirs apparent.
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