genetic probability

I'm not a student but a learning support assistant and something was taught in a lesson I was in today that makes no sense to me, I want to either be able to understand why it is true so I can help the kids understand or to know it is false so I can make sure the kids are being fed wrong information

basically we were looking at genes and the question was: cystic fybrosis is a recessive gene , a man has cystic fybrosis (so has two faulty genes) and a woman is a carrier (so one faulty one normal gene) and they have two children who are both carriers... they have a third child, what is the probability the child has cystic fybrosis

now my understanding was that the probability of each child having the disorder would remain (at year 9 biology level at least) fairly constant so each child would have a 50/50 chance regardless of how many children were born (in the same way as you pretty much have a 50/50 chance of flipping a coin and getting heads even if you just got heads 10x in a row) but the teacher insisted it was a 75% chance?

I'm not a student but a learning support assistant and something was taught in a lesson I was in today that makes no sense to me, I want to either be able to understand why it is true so I can help the kids understand or to know it is false so I can make sure the kids are being fed wrong information

basically we were looking at genes and the question was: cystic fybrosis is a recessive gene , a man has cystic fybrosis (so has two faulty genes) and a woman is a carrier (so one faulty one normal gene) and they have two children who are both carriers... they have a third child, what is the probability the child has cystic fybrosis

now my understanding was that the probability of each child having the disorder would remain (at year 9 biology level at least) fairly constant so each child would have a 50/50 chance regardless of how many children were born (in the same way as you pretty much have a 50/50 chance of flipping a coin and getting heads even if you just got heads 10x in a row) but the teacher insisted it was a 75% chance?

What was the reasoning behind the 75% chance?

Regardless of whether the first two children had CF or not, the probability of the third having CF remains 50%, independent of previous results.

If we call the recessive (CF causing) gene f and the dominant (not CF causing) gene F, then the father has ff and the mother has Ff. Therefore, the 4 possibilities for children - each having a 25% chance - are Ff, Ff, ff, ff. ff causes CF, so the probability of a child having CF is 25% x 2 = 50%. So in this situation, the chance is indeed 50%.

Yes it's an independent event, having or not having a child with CF does not affect whether the next child will have one, so I agree I think it should be 50%.

I'm not sure if it's mendelian.There might be a different inheritance pattern.I attached an example of eye color inheritance in drosophila (fruit fly).In male drosophila y chromosome doesn't carry genes related to somatic traits/characters.But if it's mendelian your teacher is wrong.

(Btw the cross is red×red and W_ phenotype is red w_ phenotype is white, red eye probability is % 75)