Some considerations:
Do you know which gene to look for?
If I understand the testing, it can only find the markers it's looking for, and may not be as conclusive as we would wish.
Even if you got a negative result, would you really feel safe, trusting your life to the result of a lab test?
If you got a positive result, would you consider that an absolute death sentence? Just because you have the gene, doesn't mean it will be expressed, depending upon the gene.
In short, I'm not sure the test would affect my behavior, I would still work, still take vacations, still marry and plan to leave a legacy. But, I admit, I would not have children if I were positive.
There are others here who understand FALS very well. Wait for them to comment in the morning.