-A curious adult from India
June 22, 2017
Fragile X is a tricky genetic disease. Given that you don’t have the disease you are either not a carrier or a sort of carrier.
In the first case your kids aren’t at any risk for getting Fragile X. You are not a carrier and so you can’t pass it on to them.
In the second case your daughters are at a very, very, very low risk of having Fragile X (your sons are at no risk). However, in this second case, your daughter’s sons have a higher risk of getting Fragile X.
This isn’t just important for gender because the cause of Fragile X is on the X chromosome (that is where the “X” comes from). Your brother has Fragile X, which means it came from your mom because he got a Y from dad. Mom is a carrier which means she has the chromosome that can lead to the disease but does not have the disease because of her other X.
Now we know your mom is carrier, but we don’t know what kind of carrier she is. There are two kinds of carriers with Fragile X—premutation and full.
Your Daughters’ Children
If a daughter ends up with the full blown mutation, then each of her sons has a 50% chance for ending up with Fragile X. (Each daughter has a 50% chance too but remember that if they have any symptoms, they would be less severe.)
It gets a little trickier if a daughter has a premutation because now there is a higher chance it can transform into a form that can cause Fragile X. Not 100% but significant. (We can’t give exact numbers because it depends on the specific premutation a person has.)