There are essentially two forms of congenital chimerism. The most common is haematopoetic or 'blood' chimerism. This occurs in non-identical twins and is particularly common in cattle. Due to the closeness of the two fetuses as they develop in the womb, an exchange of blood cells can occur, including blood stem cells. This results in the organisms having stem cells from their twin, which can then develop into certain types of cells. Chimeric cells are found in the liver, spleen and lymph nodes of the organisms. (Although a similar exchange presumably occurs with identical twins, as their DNA is already identical, the cells would not be detectable. In cattle, this can result in freemartins, female calves that are born with partially masculinized genitals, specifically with an underdeveloped vagina and uterus. Freemartins are, of course, sterile. Rarely, they occur in other livestock species, and have been reported in sheep and goats. Human freemartins have not been reported. Blood chimerism is rare in natural human twins, but is somewhat more common in multiple births resulting from in vitro fertilization. However, many cases may go unreported, especially prior to the routine testing of blood type (a blood chimera will often have more than one blood type).
The second, much rarer, form of chimerism is tetragametic chimerism. This is caused when two eggs are fertilized and then fuse completely in the womb. These chimeras tend to have the two cell lines concentrated in different organs or parts of the body. For example, they may have one set of cell lines in the kidney and another in the spleen. If the two fetuses were of opposite sex, this can cause certain forms of hermaphroditism, including the possession of both sets of sex organs, one teste and one ovary, or indeterminate tissue that falls between ovary and testes. However, this is extremely rare, indicating that the majority of 'hermaphrodite' chimeras have only one cell line expressing in the genitals. It is considered possible that some transgendered or gender dysphoric individuals may in fact be chimeras, with the sexual characteristics expressing from one line and the brain from the other. Tetragametic chimerism was once considered to be almost vanishingly rare in humans, with only those individuals that had physically obvious effects such as ambiguous genitalia, heterochromia (eyes of different colors, although this trait can also be inherited) or visible Blashko's lines causing a mosaic color effect in the skin. However, the growth of DNA testing has caused more chimeras to be detected.
In one high profile case, a woman, Lydia Fairchild, failed a maternity test. She was told that her three children could not be hers as there was no DNA match. She faced a lengthy court battle and was threatened by social workers, facing the possible loss of her own offspring...that she knew she had conceived and carried. Another woman, Karen Keegan, was also accused of fraud after genetic tests to attempt to match a kidney. In both cases, DNA matching the children was eventually found. These cases indicate that there could well be more human chimeras and that most go through their lives never knowing. Tetragametic chimerism also occurs in other animals, but likely often goes unnoticed. It can, however, be very obvious, as in the case of this horse, Dunbar's Gold. His spectacularly striped coat is a result of chimerism, however, any offspring will inherit only one cell line and be solid colored.
So, that covers the two kinds of congenital chimera. However, it is worth touching on another genetic factor that is often mistaken for chimerism. Some organisms are what is known as 'mosaics'. A mosaic only has one single cell line and is a natural individual. However, the mechanism that causes only one of the two chromosomes in a pair to turn on does not function quite correctly, resulting in some cells having one chromosome active and some the other. The majority of mosaics also likely go unnoticed, however, it is not at all uncommon for mosaicism to affect the skin color. The brown and red horse on this page, Miljon, is a mosaic. Mosaics are far more common in some species than others. In fact almost everyone has seen at least one mosaic in their lives. All tortoiseshell and calico cats are mosaics on the X chromosome, where the gene that determines between 'red' and 'dark' hair in cats is carried. This, of course, is why there are virtually no male tortoiseshell cats.
Chimeras are fascinating, and more common than most think. Some are spectacularly obvious, but many may go completely unnoticed.