Two-Faced Cats And Piebald Peacocks: The Genetics Behind Chimeras And Mosaics

Venus the cat has captured attention recently, with pictures of her starkly divided face -- black with a gold eye on one side, orange with a blue eye on the other -- spreading like only cat pictures on the Internet can.

This kitty's dramatic split personality is likely due to one of two genetic factors: mosaicism or chimerism. Chimeric animals have a mixture of genetically distinct tissues that arise from two or more fertilized eggs that fused together in the womb -- in some cases, meaning two twins have fused together into a single creature -- while mosaics have different populations of cells arising from a single fertilized egg, usually as a result of some sort of mutation.

The only way to be sure if an animal is a chimera or a mosaic is to perform a genetic test.

Chimeras and mosaics are seen throughout the animal and plant kingdoms.

Just look at this mouse:

And this horse chestnut:

And possibly this peacock (it's possible this animal could be a chimera or a mosaic, or it could be colored due to mutation, like a calico cat):

A chimera's organs may have different chromosomes, such as a heart derived from one fertilized egg, and a liver from another.

Interestingly, in the marmoset -- the tiny South American monkey -- nearly all pregnancies result in fraternal twins, and nearly all twins are chimeras, with each marmoset containing some stem cells from his or her sibling. A trio of University of Nebraska researchers proposed an explanation for this phenomenon in a 2007 paper published in the Proceedings of the National Academy of Sciences.

They found that marmoset fathers carried chimeric twins around more than non-chimeric twins, while the reverse was true for marmoset mothers. The researchers think that chimeric marmosets give off more odors that grab dad's attention.

"This changes how we think of marmosets as individuals, but it also changes how we think of the term at all," study author Corinna Ross told the New York Times. Since many male marmosets have chimeric sperm, any of his offspring would be genetically linked to his brother. "But most of his body also has his brother's genes. So what is he as an individual?"