Guide for patients

In the case of monogenic disorders our work in the PGD laboratory often uses linked DNA markers to distinguish 'healthy' from affected embryos. To find out which markers we should use we compare healthy and affected individuals within the family from at least two generations.

• To do this we therefore need your samples (from the prospective parents) and also samples from the parents of the prospective parent who is affected or a carrier, or from your children.
• Sometimes (we also) ask for blood or DNA samples from brothers and sisters, or a DNA sample from chorionic villus sampling or amniocentesis or from a fetus after a terminated pregnancy. 
• Finally a semen sample is sometimes requested for marker tests if this is necessary. Each of the requested samples (except semen samples) must be accompanied by a laboratory report (a 'protocol'), indicating whether the mutation is present or absent.

Once found, the DNA markers that can distinguish healthy family members from those who are 'sick' or 'carriers' are referred to as informative markers. These allow us to make an efficient and reliable diagnosis on the basis of the embryo cells.

It is not possible to predict whether your genetic material will be informative for the markers that we already have (see above). In addition the PGD test (on the embryo) cannot be compared with the test used to find the mutation (for you both or your family members). If a mutation is known and has been described in a laboratory protocol, the process of making a diagnosis based on blood or chorionic villi can be completed quite quickly. This is because we have a lot of DNA (from the samples requested). In the embryo, however, we must be able to detect the disorder in one or two cells. This is an extremely small amount of DNA: the PGD test must therefore be very sensitive. This gives rise to a number of time-consuming difficulties, and ultimately a new disease may be involved for which PGD has never been done before. In that case we have to start from scratch.