Genetic testing of the parents recommended - why?

Question

We recently got the results of the genetic testing of our baby - two different mutations.
Now it says at the end, that a genetic testing of the parents, including counselling is recommended. Why?
Actually it is clear, that she must have one mutation from me and the other from my husband. Is it important from whom she has which one?
Does this have a consequence for a further pregnancy?

Answer

Hello,
you report that in your child, that suffers from CF, 2 different mutations have been found. At the result sheet it has been said, that a genetic testing of the parents and a counselling should be done. You want to know why this is necessary, if in general one mutation comes from the mother and the other from the father. In the general case this is, as you said, like this. In case 2 different mutations have been found in the genetic testing of a child, it is in most cases like this, that each of the found mutations comes from one chromonsome number 7 of the chromosome pair. In very rare cases however, it can happen, that both mutations are on one chromonsome.
Then, there are two possibilites: 1. The other chromonsome carries no mutation. In this case your child would not be ill, as the "healthy" chromonsome No. 7 dominates over the defect chromonsome with the two mutations on it. 2. The other chromosome carries a mutation, that has not been detected by the genetic test (as not all possible mutations can be detected by a test), but nevertheless is a "CF-causing" mutation and contributes to the clinical picture of the child. Therefore, an investigation of both parents in order to find this out and to find out which parent carries which mutation is recommended. This is important if family members of each parent wish to have a genetic investigation/ counselling and also very important for the prenatal diagnosis in case of a further pregnancy, as if both mutations would be found in the fetus and it is not know whether they are on the same chromosome, one cannot predict for sure, if the child will suffer from CF. For explaining all details about this and also the potential problems that might occur, a genetic counselling would be helpful.
Yours sincerely,
Dr. H.-G. Posselt

25.10.2011