Inheritance of fragile X syndrome: an hypothesis.

The fragile X (fra(X), or Martin Bell-MB) syndrome is considered an X-linked recessive trait. However, clinically normal male transmitters of the condition have been observed occasionally. The occurrence of "carrier" males and the observation of other unusual genetic characteristics in the MBS suggest that this condition is not a standard X-linked recessive trait. We propose that the MBS is due to a transposable genetic element which can exist in 3 different chromosomal states and effect 2 different extrachromosomal environments. This model can account for the peculiar genetic behavior of the fragile X syndrome.