This is undoubtedly the easiest question to answer. Because fragile X syndrome is clearly seen on genetic tests and autism is not (at least, not yet) doctors can tell a parent straight up if their child has fragile x or autism. Let’s break it down a little farther.
Fragile X syndrome is called fragile x syndrome because boys’ genetic codes are made up of X and Y chromosomes, and the last pair of X chromosomes is missing some of its genetic material. Girls never get Fragile X syndrome because their genetic codes are XX and are fully developed in utero. At eight weeks, all fetuses are girls; this is a scientific fact. Something happens at this time that triggers the brains and bodies of some fetuses to develop into males. Scientists have determined that the changes occur because high levels of testosterone flow from the mother to the infant, altering their child completely. At this point, part of the transition doesn’t quite make it all the way and fragile X babies lose a small part of the final genetic chromosome that makes them male.
On all levels the babies are male, but the missing piece of the “fragile” x predetermines that they will have some mental retardation, problems with physical development, and speech and language issues. Although it might look something like autism, a genetic test will show with obvious results that it’s fragile X. If fragile X isn’t present in the genetic tests, then doctors can assume it’s another disorder, like autism. Also, fragile X will present itself a lot sooner than autism does. Fragile X is always present at birth because it began so much earlier in utero, whereas autism did not and does not.
Again, it is possible that a boy with fragile X syndrome can have autism too, because of the genetic damage caused to the brain from fragile X. However, when fragile X is diagnosed, it’s a foregone conclusion that that is the only diagnosis the little boy has, and working through that is enough. Families who have one son with fragile X have double the odds of having another son with fragile X because of the fact that both parents have to carry the gene to produce the first child with fragile X. Because it dominates the male genetic pattern in the union of a couple, they risk passing it on to all of their children, and while girls cannot develop fragile X in utero, they are carriers and can pass it on.
As for autism, there appears to be some possible genetic link, but it hasn’t been clarified or verified yet because there are no tests for it.