Investigation of the segregation of the fragile X mutation in daughters of obligate carrier women.
Two reports have suggested that over 50% of the offspring of obligate carrier women receive the mutation for the fra(X) or the Martin-Bell syndrome [Webb et al, 1986; Fryns, 1984]. Such a segregation distortion is difficult to assess for the fra(X) syndrome because of incomplete penetrance, variable expression and probable ascertainment biases. We have attempted to evaluate this possible segregation distortion in daughters of obligate carriers in a large sample of sibships ascertained in a survey of New South Wales, Australia. We used two definitions of expression: 1) presence of fra(X) positive cells if daughters were tested cytogenetically, and 2) mental impairment if daughters were not tested cytogenetically. The segregation frequency was estimated in different types of sibships of obligate carriers based on the way they were ascertained. This was done in order to have an internal check on possible ascertainment biases. Among the 189 cytogenetically tested daughters, 81 were fra(X) positive. Among the 97 untested daughters, 24 were mentally impaired in some way. Therefore, the segregation frequency as defined by fra(X) expression and/or mental impairment was 37%. Thus, no evidence was detected for segregation distortion. These data were significantly different than those collected by Webb et al [1986] and scored by the same method as the present data set.