Hi, Simon —
There is nothing in the Catholic faith that opposes genetic testing, unless it is done for the purpose of aborting a child.
If you have a serious genetic problem, you might want to be tested to see what else you have,
but did know that genes are not always expressed. I guess what you want to know is if she carries
a recessive trait that might join with one of yours, so that you would know if your children would be affected. That is a reasonable request. Perhaps your fiancée could be tested just for the worrisome genes. That would be a good compromise.