What are the four classes of fragile X repeats?
• Normal: A normal number of repeats ranges from about five to about 40. Within this range, the number of repeats stays the same when the gene is passed on to a child. The child would not have fragile X. • Intermediate: Between 41 and 58 repeats is a “gray zone.” Fragile X genes in this range are less stable. When the gene is passed on, the number of repeats may increase, but usually not enough to cause a problem. The child would not have fragile X. • Pre-mutation: When the number of repeats ranges from about 59 to about 200, the gene is in a state of “pre-mutation.” The child would not have fragile X, but the number of repeats could increase when the gene is passed on to the child. A female who carries a fragile X gene of this size is at higher risk for premature ovarian failure. • Full mutation: When the number of repeats is greater than 200, the gene cannot work properly. A boy who has a fragile X gene with more than 200 repeats will have fragile X syndrome. A girl with more than 20
