Chimerism occurs when an organism has at least two different sets of DNA. There are a variety of forms, some of them very common. For example, microchimerism, where a few cells of a different organism are found within the individual’s body is common for brief periods of time after blood transfusion and in women who have had at least one child. In fact, cells belonging to a child have been found in a woman’s blood as long as thirty years after the original pregnancy. Presumably, this form of microchimerism is found in other animals.
‘Blood’ chimerism is a congenital form found only in fraternal twins, and caused by the exchange of cells between twins in the womb. In most cases, this form of chimerism has no effects, but in cattle it can result in the birth of a ‘freemartin’. A freemartin occurs when a cow carries both a bull and heifer calf as twins. The heifer is born with normal female genitals, but with masculinization of the internal reproductive tract. This results in a stunted uterus and absent or non-functioning ovaries. Needless to say, freemartins are infertile and are generally sold for beef production. In over ninety percent of cases of cross-sex twins in cattle the female is a freemartin. (Freemartins have also been reported in sheep and goats, but far more rarely, and are unknown in humans).
Tetragrammatic chimerism results from the fusion of two embryos. It is this form that creates the more interesting and sometimes spectacular chimeras. Chimeras should not be confused with mosaics. A mosaic is an organism in which the mechanism that keeps one of each chromosomal pair turned off works incorrectly, resulting in different chromosomes being turned on in different cells. The most familiar example of a mosaic, and one almost everyone encounters, is the common tortoiseshell cat.
Most true chimeras go unnoticed. For example, they might have one type of DNA in their liver and another in their spleen. Some chimeras have only been discovered on genetic testing. At least two women found out they were chimeras after a DNA test failed to match them with their own children. However, chimerism can result in hermaphroditism. This might take the form of one testicle and one ovary, or even a full set of both.
The most obvious chimeras, however, are those in which the chimerism affects skin or hair pigmentation. In humans, this results in visible Blatchko’s lines…patterns in the skin. We all have them, but as the patterns are the same color, they are not visible. There is a good example of Blatchko’s lines here.
In furred mammals, visible chimeras can be even more spectacular. This horse, Dunbars Gold shows a pattern similar to brindle dogs, but is tested as a chimera. In this case, both gene sets code for the same color, chestnut, but one is distinctly darker than the other. However, some odd patterns are the result of somatic mutation, where a genetic mutation affects only some of an animal’s cells. This golden retriever, Seth, might appear to be a chimera, but testing indicates only one line present.
Chimeras are not mythical. They can be very strange, with odd patterns or markings and sometimes even hermaphrodites. The vast majority, however, may remain unknown until something happens to necessitate DNA testing.
