Dad's dog thinks he is dominat over me

I have divorced parents, and I lived with my mom during the first two years before she moved to japan. I decided to stay with my dad so I can finish off school.

He has a two year old mastiff-Rottweiler mix that is unneutered and used for breeding. I have never meet the dog in the past because I lived in another county.

When my dad is home he acts nice to me, but when my dad works night shifts,he thinks he owns me. During the first night alone with him, I was in my room putting things together when he came in and tried to mount me.

I'm a small girl, so I couldn't get away with all of his weight on me, and after he came on my sweat pants, he peed all over the floor. Now when ever I try to have a friend over in my room, he barks and bites at them.