In His Will and Testament, Abdu'l-Baha formally establishes the Institution of the Guardianship, and on page 11 of that document designates Shoghi Effendi as the Guardian of the Cause of God.
In a number of passages in his writings, Shoghi Effendi specifies that the Guardianship is a hereditary institution. For example, he writes that the Will and Testament "establishes the institution of the Guardianship as a hereditary office." (God Passes By, p. 327) This is in keeping with the statement of Abdu'l-Baha that Shoghi Effendi was to designate the first-born of his "lineal descendants" (The Will and Testament, p. 11) Abdu'l-Baha specifies certain spiritual qualities that must be possessed by the successor Guardian, and states that if the first-born son of Shoghi Effendi did not possess these qualities, then Shoghi Effendi must select another of Baha'u'llah's male descendants:
"Thus, should the first-born of the Guardian of the Cause of God ... not inherit of the spiritual within him (the Guardian of the Cause of God) and his glorious lineage not be matched with a goodly character, then must he, (the Guardian of the Cause of God) choose another branch to succeed him." (The Will and Testament, p. 12)
The Arabic word here translated as "branch" is ghusn, the plural of which is Aghsan. These words were not applied to anyone but the male descendants of Baha'u'llah. There was never any doubt that Shoghi Effendi could not designate anyone but a male descendant of Baha'u'llah as his successor Guardian. Shoghi Effendi himself wrote, through his secretary,
"The term Afnan means literally small branch, and refers to the relatives of the Báb, both men and women. As the Báb's only son died while in infancy, the former had no direct descendants. The Afnan are, therefore, all indirectly related to the Báb. As to Aghsan, it also means branch. But is a bigger branch than Afnan. It refers to Bahá'u'lláh's descendants."
(From a letter written on behalf of Shoghi Effendi to an individual believer, September 25, 1934; Lights of Guidance, p. 470, #1548)
This letter expressly confirms Shoghi Effendi's statement that the Guardianship is a hereditary office, that could only be held by a descendant of Baha'u'llah. Some self-seeking souls, entirely unrelated to the family of the Manifestation of God, have tried to claim that they hold the office of Guardianship. This is sheer sophistry. To hold this view is to entirely ignore the requirement that the Guardianship is a "hereditary office." Likewise, to state that the spiritual requirements of the Guardianship are more important than the physical requirement to be a male descendant of Baha'u'llah, is entirely without support in the Baha'i Writings, and is merely to engage in sowing confusion.