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PGT is presently applicable to couples who, although they may themselves be noncarriers of a mutation, have been found to have a DNM in their gonads although there is no family history of the genetic disease, or the disease is first diagnosed in one of the parents or their affected children (see Figure 2.2). As neither the origin nor relevant haplotypes may be available for tracing the inheritance of such mutations in single cells biopsied from embryos or in oocytes, the main emphasis is on the identification of the mutation and/or relevant haplotypes enabling mutation detection. Accordingly, PGT strategies for DNM depend on their origin. DNA analysis of the parents and affected children prior to PGT is required for verification of the mutation and polymorphic markers through single sperm testing and polar body analysis, thereby providing the normal and mutant haplotypes to trace the mutation. If the origin of the mutation is paternal, confirmation is first sought on the paternal DNA from blood and total sperm, and then by single sperm typing to determine the proportion of sperm with DNM and relevant normal and mutant haplotypes. It is also useful to test the relevant linked markers for the partner, to exclude misdiagnosis due to possible shared maternal and paternal markers. Where the origin of the mutation is maternal, polar body testing is the method of choice, providing the normal and mutant maternal haplotypes. Again to exclude misdiagnosis caused by possible shared paternal and maternal markers, the relevant paternal haplotypes are established through a single sperm typing. If the mutation was first detected in children, both the maternal and paternal haplotypes are established as described.

The other important phenomenon detected in PGT for DNM is gonadal mosaicism, which can be detected in either parent. Although the strategies may differ depending on the type of DNM inheritance, the general approach involves the identification of DNM origin and search for a possible gonadal mosaicism and relevant parental haplotypes.

Despite the complexity of PGT for DNM, these strategies may be applied in clinical practice with extremely high accuracy without the traditional requirement for family data, which is not always available. Since the report of our first systematic experience of PGT for 152 families with different genetic disorders,⁷³ we have performed 526 cycles from 283 couples for 81 different de novo conditions, resulting in 270 clinical pregnancies and the birth of 234 unaffected children, with no misdiagnosis.⁴⁸