Even though it sounds like a pretty weird story, it is true. This happened to a couple who went through IVF treatment to have a baby. The couple had a baby with the method of purifying and concentrating the sperms and releasing them directly to the uterus. When the baby’s blood type turns out different from the parents, the couple was confused and they got a standard paternity test. When the first DNA test didn’t match with the father, the IVF center requested a more comprehensive test. As a result of the test, the father was born with “chimerism”, which means that he carries the genes of his unborn twin.
Chimerism occurs when two separate eggs that are fertilized by two different sperms create two embryos (dizygotic twins) and these two fuse during the early stages of development (blastomer and gastrula stages) and result in a single living organism.
Chimeric organism carries cells and tissues of both twins. Born chimeric person carries the unborn twin’s DNA too. In such children, the DNA profiles of the skin and the blood doesn’t match. Since the fusion occurs during the embryo stage, tissue or organ rejection is not possible.