The strength of association between fragile(X) chromosome presence and mental retardation

In order to obtain a quantitative estimate of the degree of association between presence of fragile X chromosome (fra(X)) and mental retardation (MR), existing data from nonretarded males were analyzed. Clearly, fra(X) occurs less frequently among nonretarded compared to MR males. However, incidence estimates for fra(X) based upon existing data hold open the possibility that there may be significant numbers of nonretarded males with fra(X). Additional analyses of data from families with a pattern of fra(X) linked MR showed: (a) the probability that nonretarded male offspring will have fra(X) is very small, and (b) the probability that MR male offspring will have fra(X) is very large. Thus, accurate prognostic decisions can be based upon prenatal diagnosis of fra(X) presence, especially in families with a pattern of fra(X) linked MR.