Rare Forms of Twinning
Most sources emphasize that "polar body" twins are a hypothesis that has not been proven. In essence, polar body twins are "half identical"-- the result of one egg fertilized by two different sperm. The resulting twins share the same genes from the mother but different genes from the father.
In the theory of polar body twinning, a woman ovulates, and as her egg matures, a portion of the egg called the first polar body separates from the egg. Because this polar body contains the same genetic material as the egg itself, theoretically it could be fertilized by a sperm cell (usually, however, the polar body just disintegrates). If both the egg and the polar body are fertilized by separate sperm and two babies develop from this event, polar body twins are formed.
To date, there are no DNA tests available to parents to test their twins to see if they are polar body twins. Many experts believe polar body twinning to be quite rare, if it exists at all. Until science catches up with theory, all we can do is imagine the possibilities.
Because twins shared the uterine environment, fraternal twins can carry genetic material inside their bodies that belongs to the other twin. Some twins even have two different blood types in their body as a result. This mixing of genetic codes is called Chimerism, and is usually harmless to both twins.
Other instances of Chimerism occur when one twin "vanishes" and the other absorbs his or her genetic material. Most of these rare individuals have no idea they ever had a twin, and are surprised to find they host more than one genetic code. Discovery Health Channel aired a show on this phenomenon called "I am My Own Twin", which detailed the problems two women faced because of their chimerism. One woman needed a kidney transplant, but her DNA did not match her relatives' DNA. Another woman's children were taken away from her because a DNA test indicated she was not their mother, but that they shared the same father (they were eventually returned to her, thankfully!).
As DNA testing becomes more refined, it is entirely possible that we will find more of these individuals living amongst us--people who look and feel normal, but who possess more than one genetic code in their bodies.
Superfecundation occurs when fraternal twins have different fathers. These twins are the most disparate genetically, since they are, in essence, "half-siblings" to one another. In this type of twinning, a woman releases two eggs, each of which are fertilized by a different man's sperm. In the early 1900's, such a case occurred when an African-American woman gave birth to twins--one African-American, one mixed-race. She affirmed that she had two lovers--one African-American, the other white.
Superfetation is a rare occurrence, because it involves the release of an egg after another egg has already been fertilized. The release of the second egg can occur up until the third week of gestation of the first fetus.
In most women, the ovulation process shuts down upon the fertilization of an egg. Some women continue to ovulate again later in the cycle; in another rare instance, a woman may have two uteri, and become pregnant in each. In these cases, twins are always fraternal, and one twin is significantly smaller than the other, and if possible, is delivered later.
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