Group Level Permissions

Hello,
I have a question regarding item level permission and already existing groups in SharePoint. The situation is like described below. I have 4 users ex. Tom, Marry, John, Jack. Earlier I created to groups A(Tom, Marry) and B(John, Jack), those 2 groups uses 1 list, and I what I want is to allow Merry to have permission to Items created by Tom, but she couldn’t see items created by group B (John, Jack). Is it possible to achieve this scenario?
Thanks!