At Hopkins, Chakravarti studied the genes of 1,000 families and some 1,500 autistic children and concluded that one gene could play a key role. The gene, Semaphorin 5A, helps guide growing neurons and connects them to the right points during fetal development. The activity of this gene is lower in autistic children, researchers found. More research is needed to better understand how the gene might be responsible for autism.
None of the new findings explains why more children are being diagnosed with autism. Genes, said Goldstein, tell only part of the story.
"The idea is there is an environmental interaction with the genetic component," he said.
But no one knows what the environmental triggers are, and Goldstein suggests they might be different in every patient.
"This does not rule out that you are born with a powerful propensity to have autism, but the severity of that autism, or whether you might actually show it, might depend on something in the environment," he said.
Autism may be inherited to some degree, but even twin studies show that not all sets of identical twins have autism. And when they do, they don't always have the same severity of the disorder.
That connection between genes and the environment, called epigenetics, might explain these distinctions, said Dr. Walter Kaufmann, director of the Center for Genetic Disorders of Cognition and Behavior at Kennedy Krieger. Kaufmann is studying identical twins to better understand how certain genes may be "turned on and off" by environmental factors.
"No matter how similar the environment of twins, no two humans are exposed to the exact same conditions," he said. "There are differences and they appear to accumulate over time."
That's of huge interest to Leaird and Fetters, whose twins, now 5, were diagnosed with different types of autism.
While John is mostly nonverbal, flaps his hands and is often fixated with putting his toy blocks in a perfect line, Sam talks nonstop and is a social butterfly in his mainstream kindergarten classroom, said Leaird.
"They've always been polar opposites in ways," she said. "It was really hard for me to believe they both had autism."
Since their diagnosis, the boys received the same types of speech and occupational therapy. But early on, they led drastically different lives. John was diagnosed with a heart defect at 2 months old, and spent his early months in the hospital having major heart surgery and being pumped with antibiotics.
"I always thought maybe that environment had something to do with turning on his autism - but who knows," Leaird said.
Leaird and Fetters don't have any scientific insight into the disorder - they didn't even know a child with autism before their children were diagnosed. While they have always wondered why one son is more affected than the other, they think genes could be at work.
"While neither one of us have family on the spectrum, I just find it hard not to believe with identical twins," Leaird said. "Both are affected by autism differently, but both are affected. Right there, it is a clear indicator."