Final answer:
The ethical dilemma of replacing a human heart with a pig's heart involves concerns about cross-species infection, the challenge of removing rejection genes, and the moral implications of genetic modification in animals.
Step-by-step explanation:
Replacing the heart of a human being with a pig's heart raises ethical dilemmas related to organ transplantation and genetic modification. One ethical concern is the potential for cross-species infections due to viral sequences embedded in the pig genome.
Another issue is the challenge of removing genes responsible for rejection reactions after transplantation. Additionally, the use of pig organs raises questions about animal welfare and the moral implications of genetically modifying animals for human benefit.