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Danielle Messer is already a mother of two, but she wants another baby pretty badly.

“Pretty badly might be an understatement,” Messer told America Tonight from her home in Kentucky. “I wanted a bigger family. I wanted a girl, really bad.”

But the 36-year-old says she’s scared of having another child because she and her two sons have a genetic disorder that affects their mitochondria – the tiny structures in cells that absorb nutrients and provide energy to the body. The mutations are passed down maternally and prevent the body from functioning normally.

Although both her sons have the genetic mutation, only her younger, 11-year-old is severely affected. Aristotle, or Ari as his family lovingly calls him, suffers from Leigh’s disease. He has an enlarged heart, low muscle tone and poor lung capacity. That means he can't speak, breathe, walk or eat on his own. One thing he can do is smile.

"I guess that makes me even feel like he’s somewhat still there," Messer explained. "Like that he’s him inside of there and not a vegetable completely."

It's unlikely that Ari will live past 17, because his heart keeps getting larger. He sometimes gets sick, and the next time that happens, Messer believes he may not make it. She’s preparing to say goodbye.

“It’s probably going to be something that messes with me for a long time,” Messer said. “I mean, it is what it is, but I don’t feel like it’s fair to keep him with this kind of quality of life.”