I used to work with girl who had Fragile X Syndrome. It is literally a broken X chromosome. This is one of those tests that is a definite yes or no.
Characteristics of Fragile X syndrome on males may not look much different than autistic-like traits, and in fact is the most common KNOWN cause of autism. It can also look like mental retardation, or anywhere in between. In males, it may have physical effects as well. Their hands, feet, ears, and testicles may grow large for their body as they finish puberty, and they can get very tall. This is not a definite.
The gal I knew was mildly retarded. She had some autistic traits. She was fairly independent, but needed an assisted living arrangement (I was a part time assistant there). She was obsessed with Michael Jackson, the Cubs, and the movie Free Willie. The was near 40 at the time.
Fragile X is genetic. This gal was one of 7 kids, and every one of them had it.
http://www.fragilex.org/