johanna - it depends on the genes
Some genes are dominant. If it is a gene that causes a medical condition, every child bearing it will get the condition. Some genes are recessive, in which case a child could be a carrier, and pass it on to another generation but they won't get the condition themselves.
But it gets more complicated when the gene is on the x chromosome, as in haemophilia.
If you are a girl and bear one haemophilia gene on an x, you will have another healthy x, the gene is recessive, not dominant, so the healthy gene wins out and you will not have haemophilia. But could carry it.
If you are a boy and have one haemophilia gene on an x, you only have a teeny Y chromosome to go with it. This does not have a gene at all of this type. So the X carried gene will cause haemophilia. Queen Victoria had this gene and passed it on to many european royals.
So prenatal screening in this case of a known haemophilia carrying female, would be to ensure no boy born.
But any girl born might in turn pass it on.
I hope this makes sense.
This kind of screening is normally only done in serious conditions like this.