CHARACTERISTICS OF THE TRANSMISSION OF THE FMR1 GENE FROM CARRIER FEMALES IN A PROSPECTIVE SAMPLE OF CONCEPTUSES

The Prospective Study of the Fragile X Syndrome is a large collaborati ve effort designed to collect prospective data on the pregnancy outcom es of individuals who carry the fragile X mutation. The goal of this 5 -year study is to obtain empiric recurrence risks and population param eters for the fragile X syndrome in order to characterize the underlyi ng mechanism of the mutation and the factors that influence its expres sion. This report presents the DNA results on the first 152 cases of f emale carriers and their pregnancy outcomes. It was found that the sex ratio of conceptuses was not significantly different from 1.0 and was not associated with mutation status. Thus, there was no evidence for selection against zygotes with full mutations. There was a significant association between the form of the mutation in carrier mothers and t he frequency of its transmission. Examination of the segregation ratio s from premutation mothers showed that there was a deficit of conceptu ses that received the fragile X mutation. The segregation ratio from f ull mutation carrier mothers did not differ from expected. Several exp lanations for this observation are discussed. Numbers of cases are too small to estimate recurrence risks; however, the general trend of the data confirm the association of recurrence risks and the repeat numbe r carried by the mother. (c) 1994 Wiley-Liss, Inc.