Germline therapy aimed at preventing the transmission of serious hereditary diseases remains controversial, primarily due to uncertainty about its long-term effects. Therefore, many call for its use to be restricted to cases in which other treatment options are insufficient. Preimplantation genetic diagnosis (PGD) offers a potential alternative for preventing the transmission of serious diseases. However, in some cases, PGD may be insufficient, as it only allows for the testing and selection of embryos. In cases where both parents share the same recessive gene defect, PGD does not offer a viable solution.

The extent to which PGD represents an equal alternative to treatment with the aid of genome editing is controversial. PGD entails, for example, the discarding of embryos that could be identified as carriers of a serious hereditary disease. This is because PGD can only select and, unlike genome editing, cannot intervene curatively. However, according to current research, a possible germline intervention would also require the subsequent performance of PGD to test whether the genes were edited successfully.

Legal and moral views on both therapies differ significantly. While PGD is widely accepted and has become common practice in most Western countries, GGE remains prohibited and highly controversial. This contrast is especially striking because although both technologies are vastly different in application, they raise similar ethical concerns and produce similar responses to these concerns.

Indeed, with respect to concerns about human dignity and physical integrity, it seems that as long as GGE involves only the replacement of defective genes with healthy versions of the same gene rather than the introduction of entirely new genes, these concerns carry no more weight than they do in cases of PGD. Similarly, both therapies raise concerns about discrimination and the potential for a slippery slope. However, as long as they are recognised for their therapeutic purposes and safeguarded by appropriate regulation to ensure they remain within those boundaries, these concerns may carry limited weight in either case.

A significant instance in which the concerns diverge is that of future implications and the Nonidentity problem (NIP). In short, the Nonidentity problem claims that if our current choices determine who will exist in the future, one cannot claim that a particular person is harmed by this choice since their very existence depends on that same choice. 

Applied to PGD, this means that the choice to implant an embryo with or without a genetic disease does not harm any particular child because the existence of that child depends on the choice to implant.

Regarding GGE, this is different because we are not choosing between existence and non-existence, but between modifying and not modifying the same embryo. To determine if GGE harms a child, several factors must be assessed. First, one must ask if GGE changes the identity of the future child. Second, one must assess whether modifying the embryo is a necessary condition for implantation. If GGE is a necessary condition for implantation, the NIP applies. If it is not, there is a potential for harm, and the balance of benefits and harms must be carefully evaluated.

 The question remains whether this distinction is significant enough to warrant the stark divergence in legislation and public opinion.