You are all probably familiar with our friend here to the left, a tortoiseshell cat. While most tortoiseshells are females and the coloration is due to X-inactivation, this one is a male and is likely a chimera, the blending of two different embryos.
The process is simple to understand: While in the womb, animal embryos of two different genotypes fuse and begin developing normally as a single organism. Depending on the time this occurs, different parts of the animal's body will be of a separate genotype, sometimes creating separate phenotypes that are noticeable, like fur color.
Interestingly, the most common type of human chimera is a blood-type chimera related to the mother-fetus relationship. Some outrageous examples have been recorded, such as the case of Karen Keegan, a Boston woman who was in need of a kidney transplant. Naturally, her three sons were tested to see if they were tissue matches so they might give her the kidney she needed. Karen was shocked when her doctors initially reported that two of her naturally-conceived children couldn't be related to her due to evidence from their HLA genes. Ultimately it was discovered that Karen is a chimera, with some of her eggs belonging to her non-identical twin that fused with her while she was in her mother's womb. Karen is a mixture of two genetically distinct individuals.
And this, an interesting paragraph from here, as a teaser on microchimerism to read the whole thing. Thought it was too interesting not to C&P:
But the story doesn't end there. There is growing evidence that chimerism in one form or another may not be so unusual at all. In fact, some researchers now think that most of us, if not all, are chimeras of one kind or another. Far from being pure-bred individuals composed of a single genetic cell line, our bodies are cellular mongrels, teeming with cells from our mothers, maybe even from grandparents and siblings. This may seem a little shocking at first. The thought of playing host to cells from other people may offend your sense of individuality. But you may have those outsiders to thank for keeping you healthy.
And, of course we must also explore the non-zoological side! The only difference with the most common plant chimeras is that they're a result of human intervention - grafts gone interestingly but horribly wrong. The goal of grafting is to use rootstock from a very healthy, but rather unattractive (or not very tasty in the case of fruits) plant to support the more delicate, beautiful specimen that would otherwise have a very weak root system (or to maintain genetic purity of the fruit in common commercial cases). Well, this to the right is +Laburnocytisus 'Adamii', a graft chimera (or "graft hybrid", though it's not a hybrid) that blends Laburnum anagyroides and Chamaecytisus purpureus. It was originally produced in Paris in 1825, most likely the result of an accident. Normally one would expect the graft material and root stock not to interact much beyond moving nutrients and water, but in this (IMHO) beautiful accident, both "parent" cell lines grow with each other, creating three different types of flowers: the yellow laburnum, the purple broom, and a chimeric coppery bronze flower. I currently have one of these in my garden, waiting for it to flower this coming spring. Since this chimera would be lost through sexual reproduction, all +Laburnocytisus 'Adamii' are vegetatively derived from that original mistake 183 years ago. And as happens with other graft chimera, over time one of the two species will begin to dominate.
For those interested, there's also the Bizzarria, a citrus graft chimera.
I'm sure there's more to say on chimeras, but that's all I feel like blabbering on about for now. I'll leave you all to ponder your own microchimerism and searching for graft chimeras to plant in your yards.