X连锁智力低下是一种已明确的遗传病。约30％家族中的男性患者有X染色体标记fra(X)(q27)。这些男性患者在青春期后有大睾丸征。尽管在其它方面体征表现正常,但如在儿童期仔细检查,可以发现有微小变化:头围轻微增加,前额和下颌突出,大耳朵和淡蓝色虹膜,X连锁突变的某些女 X-linked mental retardation is a clear genetic disease. Male subjects in approximately 30% of families have the X chromosome marker fra (X) (q27). These men have large testes after puberty. Although the signs were normal on other occasions, minor changes were found during childhood: a slight increase in head circumference, prominent forehead and lower jaw, large ears and pale blue iris, some of the X-linked mutations