Why is the probability of inheriting Cystic Fibrosis 25% if both parents are only carriers? How did you get that figure?

(This would be after I have introduced the punnett square - I will draw this out for the student) Remember that each parent contributes one allele for a given gene to their offspring. For cystic fibrosis we know it's recessive inheritance, meaning the offspring needs 2 copies (of the faulty allele) to have the disease. Lets use C for the normal allele and c for the faulty allele. Fill in the punnett square grid - lets put mother along the top (Cc i.e. carrier) and father down the side (Cc i.e. carrier). How many ways can we mix the parents' alleles up? --> CC (normal), Cc (carrier - having inherited the faulty copy from the mother), Cc (carrier - having inherited the faulty copy from the father), cc (disease). This means 1 of the 4 possible combinations will cause the disease. Because all 4 combinations have an equal chance of 'ending up' in the baby, when the couple conceives they have a 1 in 4 chance of having a child with CF i.e. 25%.