Anything you write in your personal statement is fair game and you may be questioned about it.
If they ask you about your interests then you would logically talk about an interest in genetics. If a candidate wasn't able to respond with some actual examples to demonstrate their interest, I'd be suspicious they weren't genuinely interested in a particular subject.
Find which research projects a particular University is involved in and research that particular field over time so you have some background knowledge if you are asked to discuss your medical interests.
Answers in medical interviews go best where you can evidence and support individual statements. Saying: 'I am interested in genetics' might outwardly be true but it looks more convincing if a person was to say 'I am interested in genetics, I tried to expand on this by becoming a member of the genetics society at school but I've also been reading about the research that has been on-going here at the University of X under Professor Y where they are using gene therapies to treat condition Z in knock-out mice'.
The second statement shows me as an interviewer this person isn't merely saying things to score in interview, they've thought about it a lot and gone to quite some lengths to research our University and the work that is being done here.
Be open to follow on questions. Suppose I am one of your interviewers and I am an intensive care nurse. I have no idea what knock out mice are or which condition you have mentioned or the clinical application of the research you have described. it is therefore logical that interviewers might ask a follow on question about these things. It's not trying to trip you up: it's an opportunity to shine.
I've written a lot of generalised interview advice on other threads also.