Fragile X syndrome is an inherited condition that is X chromosome linked, but is recessively inherited. It occurs with a frequency of 1 in 3,600 boys and a frequency of about 1 in 5,000 girls. In other words, there can be an apparently healthy female that is a carrier and she can have a son who gets the disease. This is a syndrome that causes mental retardation, a number of malformations in the head, heart, musculoskeletal system and also causes autism. Fragile X syndrome boys often have an elongated face with large ears; their testicles are large.
These children have a social anxiety meaning that they avoid social contacts and avoid eye contact. This makes it difficult to form relationships with peers. 75% of males have excessive shyness and 50% have panic attacks. Intellectually males are more challenged than females as females have the other X chromosome that mitigates the effects of the fragile X chromosome. In a study where normal siblings and fragile X children were compared it turned out that the intellectual learning rate was 55% slowed in the fragile X children as compared to their normal siblings. Depending on how severely the child is affected the intelligent quotient (IQ) in males can span from severe intellectual disability to below average functioning to almost normal. In females who are less affected the IQ can be normal or borderline with a learning disability.
Here is an image of a child with fragile X syndrome (thanks to www.specialchild.com for this image). This image shows the genetic transmission (thanks to commons.wikimedia.org for the image) of fragile X syndrome: For more detailed information see the references below.
2. Goldman: Cecil Medicine, 23rd ed., copyright © 2007 Saunders, An Imprint of Elsevier: CHROMOSOMAL DISORDERS