There are, in fact, three types of chimerism. Blood, or artificial, chimerism, occurs by the introduction of second cell line by, for instance, transfusion. Transplacental chimerism occurs where individual cells transfer between siblings in the placenta.
It is the third type, tetragametic chimerism, that interests us here, someone who has at least two different genotypes which each arose from an individual zygote and eventually fused, when normally they would have developed separately as twins.
The individual therefore has both testicular and ovarian tissue, if the zygotes are of opposite sex. These may be arranged laterally, bilaterally or unilaterally. The most common is lateral with testicular tissue on one side and ovarian tissue on the other. However, in some cases, both sides may have testicular and ovarian tissues as ovotestis (bilateral) or in the unilateral case, both tissues are present on one side, and only one tissue is present on the other side.
Since they almost always have a penis, their situation may not be externally apparent, and most true hermaphrodites are raised as males. However, while sperm remain virtually undeveloped, they almost always also have a uterus and many undergo menstruation or ovulation.
Unrelated to intersex conditions is a rare complication known as "fetus in fetu" where one twin developes with the other inside it. Reuters reported that in Calcutta, doctors removed a fetus weighing one kilogram from a six-month-old boy (who himself weighed only 6.5 kilograms) while elsewhere doctors removed a fetus weighing 230 grams from a 40-day-old infant.
Bland.J. (2003) About Gender: Chimerism.
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01.11.03 Last amended 26.06.05