The finding of an intermediate allele in a child with a developmental disability most likely does not explain his or her symptoms, and alternative explanations should be considered. The overwhelming majority of individuals found to have intermediate alleles, including many hundreds of men and women identified through routine carrier screening, report no history of learning or developmental problems.
Studies have shown that males and females with intermediate alleles produce normal amounts of Fragile X protein, in contrast to those with full mutations who lack this protein and have FXS. That being said, there have been a handful of published reports of male and female children with FMR1 intermediate alleles having behavioral and developmental symptoms that overlap with those in FXS.
Because autism, developmental delay and ADHD are relatively common in the general population and can stem from numerous other genetic and non-genetic causes besides Fragile X, it’s likely that the finding of intermediate alleles in these children is coincidental. In fact, a large 2018 study involving 35,000 children did not find any increased rate of FMR1 intermediate alleles or premutations in children with developmental disorders as compared with children in the general population.
While the presence of an intermediate allele has not been linked to FXPOI or infertility, a small number of research papers have suggested a possible association with FXTAS symptoms in older adults. Additional research is needed to confirm any potential connection between intermediate alleles and these findings.
Also see: Making Sense of Multiple Diagnoses