Fragile X syndrome
Is there variable expression of fragile X syndrome?
Not all individuals with fragile X syndrome will be affected to the same degree. Individuals who are not making any FMRP protein are generally more severely affected and often present with moderate intellectual disability, autism-like features, and more severe anxiety and ADD. Individuals (male or female) who have more FMRP protein present due to partial methylation (only partial turning off of the gene) may show less signs and have less severe symptoms of fragile X syndrome. Additionally, females, due to the presence of a “good” X chromosome that has an FMR1 gene that is working (turned on), may also be less severely affected and only present with mild learning disabilities. Due to the way that fragile X syndrome is inherited, males with fragile X syndrome tend to be more severely affected than females with fragile X syndrome.
McLennan Y, Polussa J, Tassone F, Hagerman R. Fragile X Syndrome.Current Genomics. 2011;12(3):216-224.