How common is fragile X syndrome?
Estimates vary on the prevalence of the fragile X syndrome, but an evaluation of school children in England found a prevalence rate of approximately 1 in 1,000 males with fragile X mental retardation (Webb, 1986). The prevalence of carriers in the general population is approximately 1 in 600 (Sherman, 1992). If a 3 percent prevalence of mental retardation in the general population is used, fragile X syndrome may account for up to 10 percent of mental retardation. It is the most common inherited cause of mental retardation known to exist. The gene prevalence shows that it is the most common mutation in the human genome that causes intellectual difficulties. Why aren’t girls affected as much as boys? The X chromosome is unique among the 23 pairs of chromosomes found in each human cell. It is one-half of the chromosome pair that determines sex. If, at conception, an embryo receives an X chromosome from the mother and a second one from the father, it will be a female. But if it gets a Y-sh
