Hi Hazeleyes,
There is nothing to fear. The fact that you are aware that one of your alters answered the therapist is a good sign.
When you go through childhood trauma sometimes it's so bad that your mind separates a part of you to deal with the trauma. It's the way you survived it. It would be like you watching someone else go through it for you. Then that gets blocked and forgotten. When you start facing the issues the alter starts surfacing. As soon as you face things you begin to integrate and become one again. Now those are simple terms from me and not at all professional sounding, but that's it in a nut shell. It's not something that's passed on. It is caused by trauma.