She looks like a yellow Lab mix. I could see her having Pit Bull, or some type of bully breed, in her makeup, due to her heavyset build and barrel chest. As a mutt, she could have some terrier (fairly likely) or even shepherd in her lineage too...
It is established in several episodes that she is a mutt, but it doesn't say what breeds are in the mix.
Truman: I wish I had a bloodhound. He could help me find what I lost.
Martha: I'm a mutt, a mix of breeds. One could be a bloodhound.