In developmental disorders or intelligence impairment with IQ <70, it is long-known that males are more likely affected than females by a factor of 1.3-1.4:1. This is partially explained by X-linked genes whose variants can cause developmental disorders. The most common monogenic cause of a developmental disorder in males are variants in the FMR1 gene causing fragile X syndrome. Currently, there are more than 100 genes known whose variants cause developmental disorders, sometimes in combination with malformations, neurological or hematological abnormalities.
Piton et al. 2013, Am J Hum Genet 93:368