Today, the UK's House of Commons approved a law that would set a regulatory framework for an in vitro fertilization procedure that would create an embryo that contains DNA from three individuals. The procedure is designed to avoid having children inherit any one of a number of severe metabolic disorders that are caused by defects in the genome of the mitochondria.

Mitochondria are small compartments in the cell that do most of the work of converting molecules like sugars and fats into ATP, which the cell uses to power most of its basic functions. Mitochondria are the evolutionary descendants of bacteria that became trapped within eukaryotic cells like our own, and they still maintain a number of essential genes in a small genome. The mitochondria, along with their genome, are provided to new embryos in the egg. Thus, any genetic defects in the mitochondrial genome are passed down directly from a mother to all her offspring.

The National Institutes of Health has a list of some of the disorders that are inherited with the mitochondrial genome. These include several forms of deafness, neuro-muscular disorders, and diseases that cause blindness, among others.

Researchers in the UK have developed a technique that allows a woman who carries a mitochondrial disease to have children with normal mitochondria. The woman with the disease donates an egg that is then fertilized in a test tube. The resulting chromosomes are then transferred to the egg of a healthy donor after that donor's own genome has been eliminated. The resulting fertilized egg now has the chromosomes of two parents (the mother and father) and the normal mitochondrial genome of the healthy donor. (An alternative approach involves transferring the chromosomes from an egg prior to fertilization and then fertilizing afterwards; the end result is the same.)

In essence, the technique allows a woman who carries a mitochondrial disease to give birth to offspring that are largely genetically her own.

The UK is the first country to formally approve this process and place limits on its use; debate in parliament largely focused on whether it might be opening an avenue for something more akin to designer babies. Still, the measure passed 382 to 128. In the US, the Food and Drug Administration is currently in the process of holding advisory committee meetings on the procedure.