I want a dog but everytime I bring up the subjecy my Parents ignore me?

Im 14 going on 15 next weekend and I really want a dog. I have two cats and i would love to have the experience for a dog, my mom says that I wont take care of him/her like cleaning up after him/her or taking her/him for a walk but i disagree. She used to tell me that no one would be home to take care of it,but now that im going to school closer to my house I would be able to stay home and train them. My mom dosent want a large dog so I would like a Beagle, we have a empty space on the side of the house that's big enough for a dog house and we could putt a small gate there so they have a place for them self. Another thing my mom is concerned is that shots and food for the dog, since we already have two cats she dosent want to pay more for the dog. But she has told me when she was a kid she used to have all these animals and now that I want a dog she says no!!