Fragile X syndrome

Strictly speaking fragile X syndrome is an X-linked recessive rather than a chromosomal disorder. There are multiple (>200) CGG trinucleotide repeats on the X chromosome. Many female carriers of fragile X have mild learning difficulties. Affected boys will have moderate learning difficulties (average IQ is 50), macrocephaly, macro-orchidism, large ears, a long face, and a prominent mandible and forehead. Down's and fragile X syndromes are the most common genetic causes of severe learning difficulties in children.