"He's happy. We call him the Little Gremlin. He loves to play tricks on people. He loves to sing. His goal in life is to make people smile," Heather Britton told AOL News.
"He's got so much love around him. We're an extremely happy family. His story is not tragic."
But to an outsider, the Brittons' story might seem heartbreaking.
Another son, Trey, was born 11 weeks early and only expected to live moments. Instead, he died six weeks after his birth in 2008, on the same day he was scheduled to receive a liver transplant. Cleared to get pregnant again, the couple was thrilled when Chase was conceived, Britton said. They were eager to give older son Alex, 13, a sibling.
Chase was also born prematurely, and he was legally blind. When he was 1 year old, doctors did an MRI, expecting to find he had a mild case of cerebral palsy. Instead, they discovered he was completely missing his cerebellum -- the part of the brain that controls motor skills, balance and emotions.
"That's when the doctor called and didn't know what to say to us," Britton said in a telephone interview. "No one had ever seen it before. And then we'd go to the neurologists and they'd say, 'That's impossible.' 'He has the MRI of a vegetable,' one of the doctors said to us."
Chase is not a vegetable, leaving doctors bewildered and experts rethinking what they thought they knew about the human brain.
"There are some very bright, specialized people across the country and in Europe that have put their minds to this dilemma and are continuing to do so, and we haven't come up with an answer," Dr. Adre du Plessis, chief of Fetal and Transitional Medicine at Children's National Medical Center in Washington, D.C., told Fox News affiliate WGRZ.
"So it is a mystery."
Chase also is missing his pons, the part of the brain stem that controls basic functions, such as sleeping and breathing. There is only fluid where the cerebellum and pons should be, Britton said.
Britton's pregnancy was complicated, so doctors closely monitored her. Deepening the mystery, she has detailed ultrasound pictures of Chase's brain during various stages of fetal development and the images clearly show he once had a cerebellum.
"That is actually a fundamental part of the dilemma," du Plessis told WGRZ. "If there was a cerebellum, what happened to it?"
Doctors found no signs of a brain bleed, hemorrhage or stroke, and no damage to any other part of his brain, Britton said. Technically, his diagnosis is cerebellar hypoplasia, which normally means a small cerebellum rather than a missing one.
Chase's case, du Plessis said, challenges "fundamental principles." And its impact is certain to reach far beyond one little boy and his family.
"It is cases like this that rally the support of the medical community, that harness the interest of other investigators, that stimulate people to try and find solutions," he told WGRZ, "and those repercussions will have an impact on a much broader population of kids."
But what the Brittons know is this: Chase eventually managed to sit up on his own, something he shouldn't have been able to do without a cerebellum to provide balance. Next he learned to crawl, first dragging himself military-style, then pushing himself upright. Now, he's learning to walk.
"He keeps going," his mom said. "He keeps picking up new things and progressing. We call it, 'Chase pace.'"
In the fall, Chase started going to a specialized preschool near his New York home three days a week.
"I'm in awe of him every day," Sharon Schultz, his teacher at CHC Learning Center in Williamsville, N.Y., told WGRZ.
"Things that, based on that diagnosis, he should not be able to do, he is doing. I mean, walking up and down the hall, riding a bike, holding a pencil or a pen to work on projects, using scissors."
Chase also loves to play on his Ipad with doting brother Alex. A team of therapists has been working with him since he was an infant, and he has a special "sensory room" at home full of lights and sounds and tactile things -- like mirrors -- to visually motivate him, Britton said. Soon, she hopes he can begin horseback-riding therapy.
"We're throwing as much at him as possible to make sure he's as stimulated as possible," she explained.
Her message, she said, is simple: "Don't give up on your kids."
"Don't believe everything the doctors say. Don't get me wrong. I love doctors. But they can be wrong. ... Chase is extremely healthy. And he's extremely smart -- his motor skills just haven't caught up," she told AOL News.
"People could view this as a tragic story. But that depends on how you look at life. You can be angry or you can appreciate what you have been given," she said.
"Chase was meant to be with us."