12th Egyptian Dynasty--Nubian

 Nubia ruled Egypt in the 25^th Dynasty, but they also ruled in the 12^th.

Amenembet 1 took the throne of Egypt around 1991 BCE, the beginning of the 12^th Dynasty, and he had a Nubian mother.

It was not at all uncommon in Egypt during this time for Nubians to marry Egyptians and have children.

This information came from  to Nubia and Egypt: 10,000 B. C.. to 400 A. D. by Larry Ross, a professor at Lincoln University, Missouri.

Much of the 12^th Dynasty was ruled by individuals with Nubian ancestry.

My question is: If it truly was not uncommon for Nubians and Egyptians to marry, how can anyone say for sure that any of the dynasties did not have a ruler with Nubian blood?