Risk of CF gene in child

Question

I am not carrying the CF gene, but the father is. How big is the chance that my child has the CF gene.

Answer

Dear madam,

As you probably know the CF gene is known as the CFTR gene. Every individual has 2 CFTR genes, one inherited from his father, one inherited from his mother. Only if a person carries a serious mutation on both CFTR genes he will he suffering from cystic fibrosis (CF). If he has a mutation on just one of the CFTR genes he will be a CF carrier. CF carriers are not ill but they have a chance of one in two to transmit this mutation to every one of their children. In Belgium and the Netherlands about one in 30 individuals is a CF carrier.

Routine genetic tests can detect a serious CFTR mutation. The disadvantages of these routine tests for CFTR are that they are not 100% sensitive. In our Northern-European population they are about 90% sensitive, since only the most frequent mutations are being examined. If 3000 individuals are being tested 100 will be CF carriers. In 90 of these 100 individuals the test will detect a mutation, in the remaining 10 individuals it will not. The chance of being carrier, even if the genetic test is negative, is thus 1/300.

In your partner it was established that he is a CF carrier probably through such routine CFTR mutation-analysis. He carries a CFTR mutation on one gene, but not on the second CFTR gen, otherwise he would be suffering from CF symptoms. If the same CFTR mutation-analysis was done in yourself and a mutation was not found, the residual chance that you still are a CF carrier will be 1 in 300 (assuming you are of Northern European descent). The chance that your partner will transmit the CFTR mutation to your child is 1 in 2, the chance that you will have a undetected mutation and transmit it to your child is 1/600. The chance that your child is carrier of a CFTR mutation is thus ½ + 1/600 = 301/600. In practical terms, this boils down to a chance of 1 in 2. The chance that your child will carry two CFTR mutations and would have CF is very small, namely ½ x 1/600 = 1/1200. As you notice, this information is quite complex. If –after reading this information- you still have uncertainties we advise that you consult a clinical geneticist at a centre for genetics. Alternatively and especially if your child has symptoms we advise you to consult your pediatrician.

Sincerely yours,
H. Cuppens and K. De Boeck

20.11.2008