It may sound like a rather simple question or even bizarre riddle, but the recent announcement that British scientist have created an embryo with DNA from 3 people has caused a bit of a stir.

How is it even possible? Why was DNA from 3 people necessary, when the DNA from 2 people has, for the most part, worked well in generating Homos sapiens and their predecessors for millions of years (Leakey 1978)? Well, this new twist on fertilization has to do with mitochondrial DNA (mtDNA) and mutations that can lead to serious health problems in humans.
Mitochondria: The Cell’s Powerhouse

The mitochondrion is a type of cellular organelle, and each cell of the body typically has hundreds to thousands of them. These microscopic organelles are the powerhouses of our cells.

Mitochondria are the sites of cellular respiration, a series of reactions that turn food energy into ATP (adenosine triphosphate) energy. ATP is like the "cellular Euro" of energy currency; a molecule that can be used to drive many types of reactions within the cell. Since most of our ATP is generated by the mitochondria, the role of this organelle is absolutely vital to the viability of every cell in our body (Campbell & Reece 2005).

What Does The Three-Parent Embryo Have to Do With Mitochondria?The DNA-from-three technique is a procedure that researchers hope may be used in the future, to produce embryos free of certain inherited diseases related to mitochondrial DNA. Mutations, or mistakes, in the mitochondria's genetic code can contribute to serious genetic disorders such as muscular dystrophy, epilepsy, strokes and mental retardation.