Chimera -

from Merriam Webster:

chi·me·ra

 noun \kī-ˈmir-ə, kə-\

Definition of CHIMERA

1  a  capitalized : a fire-breathing she-monster in Greek mythology having a lion's head, 

        a goat's body, and a serpent's tail 

    b  an imaginary monster compounded of incongruous parts 

2  an illusion or fabrication of the mind; especially : an unrealizable dream <a fancy, a 

    chimera in my brain, troubles me in my prayer — John Donne> 

3  an individual, organ, or part consisting of tissues of diverse genetic constitution   

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In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information (source). 

Unless that organism happens to be a chimera (definition #3 above). 

A chimera is composed of two or more different populations of genetically distinct cells (source). In other words, some of the tissues in a chimera are of one genome, others are of one or more other genomes.

A chimeric mouse with its offspring (source)

So a chimera might have its brain, heart, and reproductive system consisting of one genome, and the rest of its body parts could all be of another genome. You might think this is a bizarre creation of someones overstimulated imagination, like the existence of a creature called the warthog (image below), but its not that at all. Chimera's are real!  Check out the eyes of the chimera mouse in the image above - one eye is of one mouse genome, the other is of a second.  

Warthog (from http://quickhidehere.blogspot.com/2011/12/lets-draw-warthogs.html)
 (Actually, Annette, warthogs are real too: some photos) 

Not only are chimeras real, but there are human chimeras too. Walking among the 7 billion of us there are millions, and possibly hundreds of millions of chimeras - individuals who are composed of 2 or more genomes.  According to this report there is a good chance YOU are actually a chimera (although, if so, likely in a quite diluted fashion).

One way one might look at a chimera is with the idea that he (or it (or they)) is/are a combination of fraternal twins - some parts (liver, heart, kidneys, brain for example) of the one twin, and the rest of the other twin. In fact this is very close to the actual make-up and of a tetragametic chimera (reference).  In the womb, sometime in the early stages of zygote development, two fraternal zygotes that would normally develop into fraternal twins, fuse instead. And then as the combined zygote continues in its cell divisions and the cells began to differentiate into specific parts and organs - some of the parts are formed using the genome of the one original zygote and others are formed using the genome of the other.  The end result is a organism that is, in part, its own twin.

this iris has two sets of distinctive DNA 

image from: an overview of chimeras

Some stories of human chimeras:

• a woman with two different blood types - apparently due to cells from her twin brother living in her body (source, and full story)
• a boy (male from outward appearances) born in 1994 who turned out to be genetically two people - one male and one female (source, and full story)
• two women who, when genetically tested, were initially determined to not to be the mothers of the children they had birthed (source, and full story)

from Venus's Page - Amazing Chimera cat on Facebook

Actually, according to lifewith cats "there is speculation and disagreement over whether Venus is genetically a chimera cat or a mosaic." (Go here for info on mosaicism and chimerism and more.)

If you want to do some further reading on this fascinating subject I recommend the Wikipedia article Chimera (genetics). Regarding the sociological implications of human chimerism, here's a good read to start with:  Which half is mommy?

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A final note - in addition to chimeras in nature there are man-made chimeras. The chimera mouse in the image at the top of this post is man-made - in that it was created by intentionally combining genetic material at the embryo or pre-embryo stage.  Recently a pair of monkeys were created which each contained 6 different genomes, see story from The Telegraph.

Although not technically chimeras, single celled organisms have been modified to contain DNA from completely different species (example:  the E. coli bacteria strain that is used to manufacture human insulin - it has a piece of human DNA that instructs it to produce human insulin, see http://www.littletree.com.au/dna.htm).

Some activities in this field of active research (modifying lifeforms) are morally questionable.  But at the same time some of the outrage and the distortions to the truth expressed by those who are opposed to these activities is also morally questionable (for an ugly sampling of that just look here).

Without the DNA modified E. coli noted above, there would be a severely insufficient supply of human insulin, and this would lead to the death and suffering of many thousands of diabetics each year.

With the transgenic studies using chimera mice, the role of specific genes in the development and function of the nervous system are being determined (Transgenic work at the National Institute of Mental Health). This type of work enables scientists to learn how disorders of human nervous systems can be affected and treated

Like almost any other human activity, research in the hands of the wrong people can become immoral.  For the most part those people will become obvious to their colleagues and they will be stopped, as they should. In rare cases others may have to step in.  Just let's not prejudge and condemn an area of research or other work (prime example: stem cell research) based on the actions of an immoral few, or based on the close-mindedness of a few who are in opposition.