Fragile X syndrome primarily affects males (1 in 1,500). Although 1 in 1,000 females is a carrier, only one in three shows outward signs of having the defect, including intellectual impairment. The range of retardation varies from mild to severe.

The physical characteristics of Fragile X syndrome may include large ears, an elongated face, poor muscle tone, flat feet, large testicles, overcrowded teeth, cleft palate, heart problems, and autistic-like tendencies. Affected children may also suffer seizures.
However, many children with Fragile X syndrome appear to be physically normal at birth, so a diagnosis may not be made until the ages of 18 months and 2 years. At that time, a lack of language development coupled with other developmental delays usually prompts testing.

As with Down syndrome, children with Fragile X syndrome can be expected to do most things that any young child can do, although they also generally learn these things later than unaffected children. And, as with most of these birth defects, early-intervention programs begun in infancy can help maximize the child's development.