One of the great science fiction movies so far this century was Alfonso Cuarón's Children of Men (2006), in which infertility threatens humanity with extinction.

(a) P-element hybrid dysgenesis in the fruit fly Drosophila is a frightening illustration of a mechanism by which this might happen in an animal species. Do some background research on this phenomenon in order to explain its molecular basis.

(b) Xenotransplantation has been suggested as a temporary solution of the problem of getting sufficient replacement organs to meet current needs. Pigs have been suggested as a potential source of organs, but most research was halted due to worries about potential risks to not just organ recipients, but to the human species as a whole from porcine endogenous retroviruses. What are endogenous retroviruses, and why do these pose a potential risk to humans.

(c) The CRISPR-Cas system for gene editing has been used to remove all known copies of porcine endogenous retroviruses from a pig cell genome. Does this suggest that xenotransplantation should now proceed (a moratorium on research was in effect until recently)? Explain your reasoning.