Every human being possesses more than 20,000 genes, the DNA instructions for our own individual blueprint. Each of them plays a crucial role in our formation, and any malfunction in that sequence can have major consequences. Case in point: A single misstep on the X chromosome is the most common cause of inherited mental retardation, also known as fragile X syndrome. This genetic disorder, which affects 1 in 4,000 males and 1 in 6,000 females, can cause developmental delays, learning disabilities, and speech and behavioral problems. Physical symptoms are subtle and include a long, narrow face and large ears.

The genetic smoking gun for the syndrome was first discovered in 1991 by three March of Dimes researchers, who named it FMR1 (fragile X mental retardation-1). The gene, which is located on a long arm of the X chromosome, may repeat in what’s called a permutation. But if this duplication occurs too many times, the gene shuts off completely. As a result, it doesn’t make the protein that it’s assigned to produce; scientists believe that this specific protein regulates communication between the nerve cells in the brain.

Because the FMR1 gene is located on the X chromosome, it’s easier for men, who have only one X chromosome, to be affected. Women must get two of the genes, one from each parent, in order to develop the syndrome. If a female inherits only one FMR1 gene, she becomes a carrier and can pass the condition on to her child. (About 1 in 259 women are carriers.) Blood tests that screen for the gene can diagnose fragile X syndrome.

ADDITIONAL FACTS

  1. Fragile X syndrome is the cause of about 5 percent of autism cases.
  2. Females with the syndrome usually have milder symptoms than men.
  3. People with fragile X syndrome may be more sensitive to light, sounds, touch, and textures.