We call genetic incompatibilities those cases where we are carriers of a disease that requires the two genes we give to the embryo be affected for the disease to manifest.
If we have the disease in only one gene, the other one can do the job and the embryo will be normal. In this situation the parents will not present the disease, but may produce embryos that are affected by a disease. This is what we call genetic incompatibility.
If both parents have a mutation in one of the genes of a disease and if they both pass the mutation to the child, the child will not have a healthy gene and will have the disease. So those parents are genetically incompatible because they can have sick children.
In such cases we do tests to see if there is any type of incompatibility. We offer the couple embryo testing in the laboratory to find out if the embryos that we have generated have inherited two affected genes and therefore will be sick. If so, we will not use them. The embryos can inherit one correct gene and one mutated gene and be carriers like their parents, but healthy, and these embryos can be used. If two correct genes are inhereited a healthy embryo is the result and that we will obviously use.