An incredible genetic circumstance creates a unique DNA puzzle.

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In genetics, a chimera is an individual formed from two different cell lines. Scientists believe that it happens when two nonidentical twin embyos (fertilized eggs) fuse into one embryo very early in their development.

Dunbars Gold, a 1996 brindle Quarter Horse stallion and a chimera

Dunbars Gold, a 1996 brindle stallion by Two D Nine and out of Outa Chiggers by Outa Utopia. Genetic testing has shown the horse to be an extremely rare chimera, an individual with two DNA types.

The embryo develops into a normal, complete individual that has two different DNA types. He or she might have kidneys that developed from one DNA type and a heart or skin cells from the other type.

Extremely rare, chimerism has been documented in other species, including cats and humans.

"Dunbars Gold has one cell line that is female and one cell line that is male," explains Dr. Cecilia Penedo. "The cell lines have slightly different DNA types, but both qualify to his sire, so there's not a parentage issue involved. It's almost clear that there were two embryos produced, and they fused."

Although chimeric in his skin and hair, Dunbars Gold's re