Is Fragile X Syndrome heterozygous or homozygous?
Fragile X carriers are heterozygous. Only females can be carriers, as it is an X-linked trait. Any male with a single mutated X-chromosome that corresponds to the mutation in Fragile X would express the disorder, whether they have a pre-mutation (50-200 repeats of the CGG sequence at the FMR1 gene on the X-chromosome, these individuals have less severe symptoms) or full mutation (over 200 repeats, these individuals have more severe symptoms). Females who are carriers may have some symptoms, but because they have one normal X-chromosome, the mutated X-chromosome is not active in all cells, they may be asymptomatic or express only some symptoms (this is due to a phenomenon called “X-inactivation”).
