Frederick "Freddy Mac" McCallum
I have a Ph. D. in genetic engineering and one in biotechnology, both from Boston University. My M.D. degree is from Harvard. For the last 20 years, I have specialized in tetragametic chimerism, a less common cause of congenital chimerism. It occurs through the fertilization of two ova by two sperm, followed by the fusion of the zygotes and the development of an organism with intermingled cell lines. This happens at a very early stage of development, such as that of the blastocyst. Such an organism is called a tetragametic chimera as it is formed from four gametes � two eggs and two sperm. Put another way, the chimera is formed from the merger of two nonidentical twins in a very early (zygote or blastocyst) phase. As such, they can be male, female, or hermaphroditic.