Intellectual disability is common. Aristaless-related homeobox (ARX) gene is one of the most frequently mutated and pleiotropic genes, implicated in 10 different phenotypes. More than half of ~100 reported cases with ARX mutations are due to a recurrent duplication of 24 bp, c.429_452dup, which leads to polyalanine tract expansion. The excess of affected males among the offspring of the obligate carrier females raised the possibility of transmission ratio distortion for the c.429_452dup mutation. We found a significant deviation from the expected Mendelian 1:1 ratio of transmission in favour of the c.429_452dup ARX mutation. We hypothesise that the preferential transmission of the c.429_452dup mutation may be due to asymmetry of meiosis in the oocyte. Our findings may have implications for genetic counselling of families segregating the c.429_452dup mutation and allude to putative role of ARX in oocyte biology.