You can't sefl-identify your etnicity because your etnicity have to be accepted by others of same etnicity in the first place. As easy as that. Not because you can't change your blood.
We're talking about blood.
Blood as important as it is important for others. Others should accept you as theirs. If they are not, even blood will not help you. If they accept you for whatever reason, blood could be not so important.
Jews will accept you as a full-value Jew if your mother was Jew. That is how they detect theirs. That is their rule. Other etnicities have other rules. There could even be cases when genetically pure person from certain etnicity could be excluded from that etnicity by others for some things he should not do, and nobody from that etnicity will account that person as theirs. And person could do nothing to change that.
You fundamentally can't self-identify etnicity, just like sex, number of fingers and many other things.
No.
You can't sefl-identify your etnicity because your etnicity have to be accepted by others of same etnicity in the first place. As easy as that. Not because you can't change your blood.
Blood as important as it is important for others. Others should accept you as theirs. If they are not, even blood will not help you. If they accept you for whatever reason, blood could be not so important.
Jews will accept you as a full-value Jew if your mother was Jew. That is how they detect theirs. That is their rule. Other etnicities have other rules. There could even be cases when genetically pure person from certain etnicity could be excluded from that etnicity by others for some things he should not do, and nobody from that etnicity will account that person as theirs. And person could do nothing to change that.