It occurs to me that there was never supposed to be a 13th tribe. Hashem's plan all along was 12 tribes of Israel. It is for this reason Dinah was born a woman. There is an interesting explanation of Dinah on the Chabad site:
One of the reasons, according to the sages, that Dinah was raped was because she was an 'outgoing' gal. She would go out and not be so modest in doing so. Jacob locked her in a chest to prevent her from meeting Eisav lest he believe he could marry her...
http://www.chabad.org/parshah/article_cdo/aid/2668/jewish/Outgoing-Woman.htmOur sages note that in the opening verse of its account, the Torah introduces Dinah as Leah’s child. She is not referred to as “the daughter of Jacob,” or “the daughter of Jacob and Leah,” or even as “the daughter of Leah and Jacob,” but as “the daughter of Leah, whom she bore to Jacob.” Rashi explains:
Because of her going out, she is called “the daughter of Leah.” For [Leah,] too, was an “outgoer,” as it is written, “And Leah went out to greet him” (Genesis 30:16). Regarding her it has been said, “Like mother, like daughter.”At first glance, this seems an indictment of Leah’s and Dinah’s behavior. The hallmark of the Jewish woman is her tzniut, the modesty in dress and demeanor expressed by the verse (Psalms 45:14), “The entire glory of the king’s daughter is within.” A Jewish girl, Rashi seems to be saying, has no business going out to visit with the daughters of a pagan land; when she does, she is not acting as a daughter of Jacob but like her mother, who is known to have—on occasion—embarked on outings of her own. For the king’s daughter to leave her inner sanctum is to expose herself to all sorts of negative encounters, as Dinah’s case tragically demonstrates.1
This, however, cannot be Rashi’s intention, for it runs contrary to what he writes in his commentary on a previous verse. A few chapters back, where Jacob is preparing for his encounter with his wicked brother Esau, we read (Genesis 32:23):
And [Jacob] took his two wives, his two handmaidens and his eleven sons, and he crossed the ford of Jabbok.Asks Rashi: What about his daughter?
Where was Dinah? Jacob had placed her in a chest and locked her in, lest Esau set his eyes on her. For this, Jacob was punished, for had he not withheld her from his brother, perhaps she would have brought [Esau] back to the proper path. [The punishment was] that she fell into the hands of Shechem.In other words, it was Jacob’s isolation of Dinah, not Dinah’s and Leah’s outgoingness, that was the cause of Dinah’s misfortune. Dinah should not have been hidden from Esau. Her encounter with the big, bad world should not have been avoided; indeed, it should have been welcomed. Jacob feared that she would be corrupted by her wicked uncle; he should have realized that, with her firm moral grounding and unassailable integrity, she was far more likely to influence Esau for the better.
Interestingly enough, here, too, there is a mother-daughter connection. The Torah (in Genesis 29:17) tells us that “Leah’s eyes were weak.” Rashi explains that they were weak from weeping:
She wept over the thought that she would fall to the lot of Esau. For everyone was saying: Rebecca has two sons and Laban has two daughters; the elder son (Esau) is destined for the elder daughter (Leah), and the younger son (Jacob) for the younger daughter (Rachel).This was more than common speculation; according to the Midrash, these were matches ordained in heaven. But Leah’s tearful prayers changed the heavenly decree, and both sisters were married to the righteous younger son. But it was Leah who was Esau’s potential soulmate. If she herself felt unequal to the challenge of dealing with his wickedness, her daughter and spiritual heir, Dinah, could have served as the instrument of Esau’s redemption.
This is the deeper significance of the adage, “Like mother, like daughter,” quoted by Rashi. Our children inherit not only our actual traits but also our unrealized potentials. Physically, a brown-eyed mother may transmit to her child the potential for blue eyes inherited from her mother but dormant in her genes. Spiritually, a parent may impart to a child the ability to achieve what, for the parent, is no more than a subtle potential buried in the deepest recesses of his or her soul.