How did Chazal know that Dinah was missing?
(32,23) “And he arose that night and took his two wives and his two maidservants and his eleven children and crossed over the crossing point of the Yabok.”
Since at that time Ya’akov had twelve children, Rashi asked: And where was Dinah? He put her in a chest and locked her in, so that Eisav should not set eyes on her.
The question is asked how Chazal knew that the one who was missing was Dinah - maybe Dinah was one of the eleven children mentioned in the posuk, and one of her brothers was missing?
But behold, the gemora writes that the reason why the Beis Hamikdash was built in the territory of Binyamin was because he had not bowed down to Eisav, since he had not yet been born at that time. Now, if one of the brothers had been missing, then he too would not have bowed down to Eisav, and if so, why was the Beis Hamikdash chosen to be built in the territory of Binyamin and not in the territory of that missing brother? It must be that all the brothers were there with Ya’akov and they all bowed down to Eisav, except for Binyamin who had not yet been born. Therefore Dinah, who in any case did not have a portion in the land, must have been the one who was missing. Hence, the question “where was Dinah?” is justified.
Was the fifth son of Seir called Dishon or Dishan?
(36,20) “These are the sons of Seir the Chorite who lived in the land: Lotan and Shoval and Tzivon and Anah. And Dishon and Eitzer and Dishan.”
Behold, from this posuk it seems that the fifth son of Seir was called Dishon (with the vowel called a cholam), and the seventh son was called Dishan (with the vowel called a kometz), and this is what it also says in posuk 30. But when the Torah enumerates the grandchildren of Seir, it says in posuk 26 “and these are the sons of Dishan” apparently referring to the fifth son of Seir according to the order of enumeration, and in posuk 28 it says also “these are the sons of Dishan” apparently referring to the seventh son. It calls both of them by the same name, Dishan, and so it apparently mentions the sons of Dishan twice but it does not mention the sons of Dishon, the fifth son, at all!
However, we can explain that Dishon in posuk 21 and Dishan in posuk 26 are one and the same. But since in posuk 26 the word has a cantillation note called an 'asnachta' (a note indicating a pause in the posuk similar to our semicolon), the cholam changes to a kometz, in the same way that certain words which have a patach change to a chometz at the end of a posuk, or in the middle of a posuk when there is an asnachta. Thus, the word for heaven - shomayim, changes to shomoyim, and the word for water - mayim, changes to moyim.
The proof for this is in Divrei Hayomim Ⅰ where it enumerates the genealogy of the children of Seir. There it writes that the fifth son mentioned in posuk 1:41 and the seventh son mentioned in posuk 1:42 were both called Dishon, because there neither word has an asnachta.
What emerges from all this is that both the fifth and seventh sons of Seir had the same name, and they could variously be called either Dishon or Dishan, but if the name Dishon is used it is vowelised Dishon in the middle of a posuk and at the end of a posuk or with an asnachta it is vowelised Dishan.
This rule explains the different vowelizations of the sons of Seir. But the grandson of Seir, the son of Anah, was always called Dishon, and therefore in posuk 25 it is written Dishon even though it has an asnachta. Therefore in Divrei Hayomim in posuk 41 it is written Dishon even though it has an asnachta, because the son of Anah was always called Dishon.