Dz. Loesch et al., EXPANSION OF THE CGG REPEAT IN FRAGILE-X IN THE FMR1 GENE DEPENDS ON THE SEX OF THE OFFSPRING, American journal of human genetics, 57(6), 1995, pp. 1408-1413
Analysis of 139 mother-to-offspring transmissions of fragile X CGG tri
plet repeats revealed that the repeat expansion is enhanced in mother-
to-son transmissions compared with mother-to-daughter transmissions. E
vidence has been based on analysis of mother-offspring differences in
the size of repeat (in kb), as well as on comparisons between proporti
ons of male and female offspring with premutations, and full mutations
, inherited from mothers carrying a premutation. Mean difference in th
e repeat size from mother-son transmissions was 1.45 kb, compared with
mother-daughter transmissions of 0.76 kb. The difference is due prima
rily to a greater proportion of male than female offspring with full m
utation from the premutation mothers and also to a higher frequency of
reduction in repeat size from mothers to daughters than from mothers
to sons. Our findings suggest the possibility of an interaction of the
normal X homologue in a female zygote with the FMR1 sequence on the f
ragile X during replication to account for the lower level of expansio
n in mother-to-daughter transmissions relative to mother-to-son transm
issions.