Is a person's ethnicity fixed

Answer 1

Within somebody's life, the answer is usually, yes. A person cannot change from being ethnically Yoruba (Nigeria) to being ethnically Thai. However, there are some circumstances where ethnicity is contingent on historical circumstances and not some fixed DNA counter.

A perfect example of changing ethnicities would be in the creation of the Turk ethnicity in Anatolia and the Balkans. By the time of Suleiman the Magnificent to the present day, the majority of Turks in Turkey and the surrounding countries had no or minimal ethnic connection to the Turks of Turkmenistan, Uzbekistan, and Kazakhstan, where their Turkic conquerors, the Seljuks and Ottomans, came from. They were actually the indigenous people of Anatolia and the Balkans, many of whom had previously identified as Byzantine Greeks, but who converted to Islam. It was this conversion to Islam that made them Turks. They were also not linguistically homogeneous. Roughly half of the Turks in Anatolia and the Balkans prior to 1900 did not speak Turkish at all. They spoke Greek, Serbo-Croatian, or Bulgarian since those were the languages of the regions where they were from. The process of indigenous Anatolians and Balkans-peoples converting to Islam and, therefore, becoming ethnically Turkish, was called "Turkification".