Will my supervisor also mark my MA dissertation? How will this be fair? I am wondering, because she is the only one in the department who is specialised in the area I worked in. But then also she worked with me, so she will recognise the things I've written - apart from that there is probably noone else who tackled exactly my topic.

How is fairness ensured here? It does not seem possible to mark this anonymously really?
On the topic of anonymous marking.  Dissertations/theses are rarely, if ever anonymised, because students and their topics are easily visible.

What you are asking is really a matter of Quality Assurance, which will depend on the assessment policy relating to Master's theses at your department/institution.

It is worth saying that the bias you mention (of a supervisor's familiarity with your work and writing influencing their assessment of it) is recognized.

In my experience, to mitigate bias one of two actions are taken:


The assessment is double-marked, once by the supervisor and once by a second marker.  Any difference in marks are then discussed and agreed.
The assessment would be marked by someone not related to your project at all, and it may also be double-marked by someone else.


For Master's theses, at least from what I have seen, the subject specialism and expertise of the assessor is less essential than it would be in doctoral study, so I would expect an assessor with good familiarity with the subject to be able to satisfactorily mark it.

Also, in the UK, all assessments are ratified by a Board of Examiners who have the power to alter marks at their judgement.  The External Examiner in this Board would also have the power to read your dissertation and agree/disagree with the mark awarded - they, and the rest of the Board, won't necessarily be experts in the subject to the degree that your supervisor is.