Now these are the names of the children of Israel who came to Egypt; each man and his household came with Jacob... All those who were descendants of Jacob were seventy persons.Who are the seventy people who came down to Egypt? Gen. 46 lists the exact names by mother: Leah's children -- 33 (vv. 8-15), Zilpah's children -- 16 (vv. 16-18), Rachel's children -- 14 (vv. 19-22), Bilhah's children -- 7 (vv. 23-25).
However, if you count Leah's children as listed in the verses, they total 34 and not 33. Rashi (v. 15) assumes that Er and Onan (who did not go to Egypt) are not counted in the total, which brings it down from 34 to 32. He then adds Yokheved, a granddaughter who was born as they were entering Egypt, which brings the total to 33. Rashbam (v. 8) and Ibn Ezra (v. 27) instead add Ya'akov to the count of those who went down to Egypt. Ibn Ezra adds that if Yokheved had been born at that early time, then she would have had to give birth to Moshe at the age of 130. If she did, why doesn't the Torah mention this miracle? On this point, Ramban (v. 15) strongly objects to Ibn Ezra's question because life is full of miracles that aren't mentioned in the Torah.
One can ask Rashbam and Ibn Ezra how Ya'akov can be counted as one of the "children of Israel" and why he would be grouped with Leah's children rather than any of the other wives or simply independently.
One can ask Rashi why only one granddaughter is counted and not any other. However, one could perhaps answer that just like Dinah was counted, presumably because she figured prominently in an episode in the Torah, Yokheved was also counted because she was important in the Moshe story. However, at the time of the descent to Egypt she was a newborn.
The Netziv (v. 27) makes an important point. Just like the Ramban points out that there are really 13 tribes (Ya'akov's twelve sons with Yosef being counted twice for Ephraim and Menashe) but the Torah always counts them as 12 but sometimes with different ways of counting the 12, so too with the 70. 70, clearly, is a significant number and the Torah wanted to use that number, even if it meant finding different ways of reaching that 70.
R. Mordechai Breuer (Pirkei Bereishis, ch. 42) is certain that Er and Onan are counted in the 70 mentioned in Ex. 1, where it specifically says that this is a count of the descendants (yotz'ei yerekh) of Ya'akov and cannot include him. Rather, it is a count of the tribes of Israel, a unit that eventually descended to Egypt. In that original unit were Er and Onan, even if they died before going to Egypt.
However, the 70 mentioned in Gen. 46 is a list of those who actually went to Egypt and therefore cannot include Er and Onan. To make up for them, Dinah and Ya'akov were added to the list so it would add up to 70.
(I'm not sure what R. Breuer does with Gen. 46:26)